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What is the Protocol Data Unit (PDU) at Layer 1 of the OSI model? empezar lección
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Which of the following transmission media belong to the category of guided media? empezar lección
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Twisted pair, coaxial cable, and fiber optic
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Why is differential signaling used in Ethernet? empezar lección
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To cancel external electromagnetic interference by subtracting the two opposing signals at the receiver
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Which duplex mode is described by the classic walkie-talkie principle? empezar lección
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What is the main problem with NRZ-L (Non-Return-to-Zero Level) encoding when transmitting long runs of identical bits? empezar lección
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Long runs of the same bit produce no signal transitions, causing the receiver to lose clock synchronization
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Which encoding scheme does 100BASE-TX use, and what efficiency does it achieve? empezar lección
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4B/5B with MLT-3, 80% efficiency
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How many discrete voltage levels does PAM-5 use, as employed in 1000BASE-T (Gigabit Ethernet)? empezar lección
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5 levels (−2, −1, 0, +1, +2)
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What does the abbreviation BASE mean in Ethernet designations such as 10BASE-T? empezar lección
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Baseband transmission — the entire cable carries a single digital signal
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A noise-free channel has a bandwidth of 5 MHz and uses PAM-4 (4 signal levels). What is the maximum data rate according to the Nyquist theorem? empezar lección
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What does the Shannon-Hartley theorem describe? empezar lección
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The absolute theoretical upper limit of the data rate on a noisy channel, regardless of the encoding scheme used
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What is the maximum supported data rate of Cat5e cables? empezar lección
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What is the maximum reach of Cat6 at 10 Gbit/s, and why is Cat6A superior? empezar lección
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Cat6 only reaches 55 m at 10G; Cat6A achieves
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How much power does the PoE standard IEEE 802.3at (PoE+) deliver to the powered device (PD)? empezar lección
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IEEE 802.3bt Type 4 delivers up to 71.3 W. Which cable category is recommended for long PoE runs, and why? empezar lección
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Cat6A or cables with larger conductor cross-sections, to minimize resistive losses over long distances
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Why can PoE (Power over Ethernet) not be transmitted over fiber optic cables? empezar lección
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Fiber is an electrical insulator (glass) and cannot conduct electric current
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What is the core diameter of single-mode fiber (SMF), and what does this enable? empezar lección
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8–9 µm, allowing only one light mode and enabling distances of 80 km or more
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Why must APC connectors (8° angled) never be mated with UPC connectors (flat-polished)? empezar lección
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The angular mismatch creates an air gap that physically damages both ferrules and causes high insertion loss
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A new 10 Gbit/s copper run of 80 m is planned (no EMI, limited budget). Which cable category is correct? empezar lección
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Cat6A — supports 10GBASE-T up to
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What is the maximum allowed untwist length when terminating Cat6A cables at an RJ45 connector? empezar lección
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In NRZI (Non-Return-to-Zero Inverted) encoding, how is a binary 1 represented on the wire? empezar lección
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A transition (level change) occurs at the start of the bit period
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What signaling scheme does 100BASE-TX place on the wire after 4B/5B block coding? empezar lección
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MLT-3 (Multi-Level Transmit, three levels)
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Why does 4B/5B coding map 4 data bits onto 5-bit code words? empezar lección
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The redundant codes guarantee enough transitions (no more than three consecutive zeros) for clock recovery
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What is the main purpose of scrambling a bit stream before transmission? empezar lección
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To randomize the data so long runs of identical bits are broken up, aiding clock recovery and reducing spectral peaks
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Differential Manchester encoding signals a binary 0 by empezar lección
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a transition at the start of the bit period (the mandatory mid-bit transition is always present)
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Why does MLT-3 reduce the dominant signal frequency compared to binary NRZI at the same bit rate? empezar lección
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It takes four bit periods to complete one full waveform cycle, so the fundamental frequency is about one quarter of the bit rate
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Which Ethernet standard uses Manchester encoding on the wire? empezar lección
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A channel has a bandwidth of 4 kHz and uses 4 signal levels. What is the Nyquist capacity? C = 2 · B · log2(L). empezar lección
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Using the Nyquist formula C = 2 · B · log2(L), how many signal levels are required to reach 24 kbit/s over a 3 kHz channel? empezar lección
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A channel has B = 1 MHz and a linear SNR of 3. What is the approximate Shannon capacity? C = B · log2(1 + SNR). empezar lección
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A 10 MHz channel has an SNR of 20 dB. What is the approximate Shannon capacity? (20 dB = linear 100.) empezar lección
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Bandwidth and data rate are related but distinct. Which statement is correct? empezar lección
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Bandwidth is a frequency range in Hz; for a fixed bandwidth the data rate rises by using more signal levels or higher SNR
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A −10 dB change in power corresponds to which multiplier? empezar lección
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What does 0 dBm represent in absolute terms? empezar lección
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Differential signaling cancels external noise because empezar lección
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the same interference is induced on both wires and the receiver outputs the difference (A − B), removing the common component
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Which duplex mode transmits in one direction only, such as a broadcast TV signal? empezar lección
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Cat6 cable is specified to what frequency bandwidth? empezar lección
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Cat8 cable is specified to which bandwidth, and what is its maximum reach? empezar lección
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2,000 MHz, with a maximum reach of about 30 m
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A standard twisted-pair data cable contains how many pairs, and why are the pairs twisted at different rates? empezar lección
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Four pairs; different twist rates reduce crosstalk between the pairs
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Why must shielded cabling be properly grounded at the appropriate point(s)? empezar lección
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Improper grounding can let the shield act as an antenna and worsen performance instead of improving it
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FEXT (Far-End Crosstalk) is distinguished from NEXT by empezar lección
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being measured at the far end of the link, where the disturbing signal has also attenuated
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Why is alien crosstalk the key barrier that Cat6A addresses for 10GBASE-T over 100 m? empezar lección
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Because 10 Gbit/s uses all pairs in both directions, and AXT from neighboring cables cannot be cancelled by the receiver; Cat6A's design and shielding suppress it
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What limits the distance and bandwidth of multimode fiber compared to single-mode? empezar lección
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Modal dispersion, as different light paths (modes) arrive at slightly different times
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Which fiber wavelength is associated with the C-band used for long-haul single-mode transmission and lowest attenuation? empezar lección
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The ST fiber connector is characterized by which coupling mechanism? empezar lección
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By color code, an APC connector body is typically which color, and UPC which color? empezar lección
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APC is green; UPC is blue
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IEEE 802.3bt Type 4 (PoE++) delivers what power at the PSE, and what is available at the powered device? empezar lección
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Up to 90 W at the PSE, with about 71.3 W available at the PD
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What does the "BASE" in standards like 100BASE-TX and 1000BASE-T denote? empezar lección
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Baseband signaling, where the medium carries a single digital signal occupying the full bandwidth
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How many pairs does 1000BASE-T use, and in what manner? empezar lección
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All 4 pairs, each carrying data bidirectionally and simultaneously with echo cancellation
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What is the baud (symbol) rate of 100BASE-TX on the wire? empezar lección
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Which Ethernet copper standard requires Cat6A to reliably reach 100 m? empezar lección
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What is the chief disadvantage of Manchester encoding that motivated its replacement at 100 Mbit/s? empezar lección
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It needs twice the bandwidth of the bit rate (only 50 percent efficient)
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Which statement about clock recovery and line coding is correct? empezar lección
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A good line code guarantees frequent transitions so the receiver can continuously recover and lock its clock to the data
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What is the Protocol Data Unit (PDU) at Layer 2 of the OSI model? empezar lección
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What is the main function of the LLC sublayer (Logical Link Control, IEEE 802.2)? empezar lección
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Protocol multiplexing: identifying the Layer 3 protocol (IP, ARP, IPv6) carried in the frame
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What do the first 3 bytes (24 bits) of a MAC address encode? empezar lección
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The OUI (Organizationally Unique Identifier) — the manufacturer identifier assigned by the IEEE
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What does I/G bit = 1 in the first byte of a MAC address indicate? empezar lección
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The address is a multicast or broadcast address (group address)
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Which MAC address is the universal broadcast address, and which protocols typically use it? empezar lección
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FF: FF: FF: FF: FF: FF — used by ARP requests and DHCP Discover, among others
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An Ethernet frame contains EtherType 0x0806. Which Layer 3 protocol is in the payload? empezar lección
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ARP (Address Resolution Protocol)
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Why is the minimum Ethernet frame size 64 bytes? empezar lección
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Due to the legacy CSMA/CD mechanism: a frame had to be long enough so that it was still being transmitted when the collision signal returned from the far end of the cable
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What does the MTU (Maximum Transmission Unit) define in standard Ethernet, and how does it differ from the total frame size? empezar lección
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The MTU is 1500 bytes (payload only); the total frame size is up to 1518 bytes (Preamble/SFD are outside the counted frame)
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What can CRC-32 (Frame Check Sequence) do, and what can it NOT do? empezar lección
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CRC-32 can detect errors but NOT correct them; corrupted frames are silently discarded, and correction is handled by higher-layer protocols such as TCP
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What is the function of the preamble (7 bytes) in an Ethernet frame? empezar lección
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It is an alternating 1-0 pattern that allows the receiver to synchronize its clock to the incoming signal frequency before the actual data arrives
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What is the fundamental difference between a hub (Layer 1) and a switch (Layer 2) regarding collision domains? empezar lección
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A hub forms one large shared collision domain for all ports; a switch creates a separate collision domain per port (full-duplex, no collisions)
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What does a switch do when it receives a frame destined for a MAC address that is not in its CAM table? empezar lección
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It floods the frame to all ports except the ingress port (unknown unicast flooding)
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Why can devices in different VLANs not communicate directly at Layer 2? empezar lección
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VLANs create separate broadcast domains; a Layer 3 device (router or L3 switch) is required for inter-VLAN communication
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Which EtherType value does an 802.1Q VLAN tag use in an Ethernet frame? empezar lección
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How long does convergence take in classic STP (802.1D), and what is the operational consequence? empezar lección
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Approximately 30–50 seconds; this represents a significant outage during topology changes
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Host A (192.168.1.10) wants to send a frame to Host B (192.168.1.20) but does not know its MAC address. Which ARP sequence is correct? empezar lección
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Host A broadcasts an ARP Request to FF: FF: FF: FF: FF: FF; only Host B replies with a unicast ARP Reply containing its MAC address; Host A caches the mapping in its ARP table
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What is a Gratuitous ARP, and for which legitimate purposes is it used? empezar lección
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An unsolicited ARP Reply in which a device announces its own IP-to-MAC mapping; used at boot time, during failover, and to detect IP address conflicts
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Which of the four core functions of the Data Link Layer is responsible for organizing a raw stream of bits into structured units before transmission? empezar lección
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How many total bits and bytes make up a standard MAC address? empezar lección
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When the I/G bit (bit 0 of the first byte) of a MAC address is 0, the address is: empezar lección
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A unicast address (a single, individual device)
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The Logical Link Control (LLC) sublayer is defined by which IEEE standard? empezar lección
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In an Ethernet II frame, how large is the EtherType field? empezar lección
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What is the valid payload size range of a standard Ethernet II frame? empezar lección
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An end station receives an Ethernet frame with EtherType 0x0800. Which protocol does the payload contain? empezar lección
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How large is the Preamble in an Ethernet frame, and what is its purpose? empezar lección
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7 bytes of alternating 1-0 pattern used for receiver clock synchronization
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What is the standard Ethernet MTU, and what does it count? empezar lección
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1500 bytes, counting the payload only
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A station needs to send a payload of only 20 bytes. What happens to satisfy the minimum frame size? empezar lección
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The payload is padded up to 46 bytes so the total frame reaches 64 bytes
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Which of the following is NOT something CRC-32 can detect? empezar lección
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It cannot correct any errors it finds
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What is the typical default aging timer for entries in a switch CAM table? empezar lección
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In a CAM table overflow attack, the attacker floods the switch with many random source MAC addresses. What is the result? empezar lección
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The CAM table fills up and the switch falls back to flooding all frames, allowing the attacker to sniff traffic
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At which OSI layer does a hub operate, and how does it handle an incoming frame? empezar lección
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Layer 1; it repeats the signal out of all other ports regardless of destination
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A bridge and a switch are functionally related. Which statement best describes their relationship? empezar lección
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A switch is essentially a multiport bridge implemented in hardware; both operate at Layer 2 and learn MAC addresses
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The 64-byte minimum Ethernet frame size is fundamentally tied to which mechanism? empezar lección
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CSMA/CD collision detection — the frame must still be transmitting when a collision signal could return from the far end
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In an 802.1Q tag, what is the value of the TPID field that identifies the frame as VLAN-tagged? empezar lección
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What is the function of the 3-bit PCP (Priority Code Point) field in an 802.1Q tag? empezar lección
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It carries a Class of Service (CoS) priority value from 0 to 7 for QoS
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On a trunk port, frames belonging to the native VLAN are transmitted in what manner? empezar lección
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Two PCs are connected to the same switch but are in VLAN 10 and VLAN 20 respectively. To communicate, what is required? empezar lección
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A Layer 3 device (router or L3 switch) with an interface in each VLAN
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Classic STP is defined by which IEEE standard? empezar lección
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How does STP elect the root bridge? empezar lección
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The switch with the lowest Bridge ID becomes the root bridge
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Which destination MAC address is used for STP BPDU (Bridge Protocol Data Unit) frames? empezar lección
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What is the role of an STP root port on a non-root switch? empezar lección
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It is the port with the lowest path cost toward the root bridge
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In which STP port state does a port populate the MAC address table but still not forward user frames? empezar lección
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Which standard STP path cost corresponds to a 100 Mbps link? empezar lección
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How does the target host send its ARP Reply? empezar lección
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As a unicast directly back to the requesting host
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What is the typical default timeout for entries in a host's ARP cache? empezar lección
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A user on an access port in VLAN 10 cannot reach a server on the same switch. show vlan brief reveals the server's port is in VLAN 20. What is the cause? empezar lección
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A VLAN mismatch — the two devices are in different broadcast domains and need Layer 3 routing
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Two directly connected switch ports show many late collisions and CRC errors, and throughput is poor. show interfaces status hints at a configuration issue. What is the most likely cause? empezar lección
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A duplex mismatch (one side half-duplex, the other full-duplex)
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Cut-through switching achieves very low latency because it: empezar lección
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Begins forwarding as soon as the destination MAC is read, without waiting for the full frame or CRC check
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Which Layer 2 attack is mitigated by Dynamic ARP Inspection (DAI)? empezar lección
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ARP spoofing (ARP cache poisoning)
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Wireshark display filter eth. type == 0x0806 would capture which traffic? empezar lección
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Link Aggregation (IEEE 802.3ad / LACP) combines multiple physical links. How does it typically distribute traffic? empezar lección
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By hashing fields such as source/destination MAC or IP to select a member link (per-flow)
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The destination MAC FF: FF: FF: FF: FF: FF in a frame tells a switch to: empezar lección
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Flood the frame to all ports except the ingress port
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An IPv4 address is structured as 32 bits. How are these bits grouped in dotted-decimal notation? empezar lección
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Four octets of 8 bits each, separated by dots
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What is the decimal value of the binary octet 00010000? empezar lección
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Which 8-bit binary value represents the decimal number 172? empezar lección
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How many total IPv4 addresses are theoretically possible with 32 bits? empezar lección
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Which IPv4 address class is reserved for multicast? empezar lección
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Which CIDR prefix corresponds to the RFC 1918 private range 172.16.0.0? empezar lección
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What is the full address range covered by the loopback block 127.0.0.0/8? empezar lección
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The address block 169.254.0.0/16 is used for which purpose? empezar lección
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Link-Local / APIPA addresses self-assigned when DHCP fails
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How many usable host addresses does a /29 subnet provide? empezar lección
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How many /28 subnets can be created from a single /24 network? empezar lección
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What is the dotted-decimal subnet mask for a /20 prefix? empezar lección
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What is the network address of host 10.10.10.130 with a /25 mask? empezar lección
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For the subnet 192.168.1.64/26, what are the first and last usable host addresses? empezar lección
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First: 192.168.1.65, Last: 192.168.1.126
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Using a /27 mask, what is the block size (number of addresses per subnet)? empezar lección
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A point-to-point WAN link needs only 2 usable addresses. Which prefix is the classic choice? empezar lección
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In a VLSM design, after allocating Engineering 10.0.0.0/25 (126 hosts), at which address does the next subnet start? empezar lección
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The IHL (Internet Header Length) field holds its minimum value of 5. How many bytes of header does this represent? empezar lección
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20 bytes (5 × 4-byte words)
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What is the purpose of the Identification field in the IPv4 header? empezar lección
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It carries the same value for all fragments of one original packet so the destination can group them for reassembly
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Which protocol number appears in the IPv4 Protocol field for UDP? empezar lección
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The MF (More Fragments) flag is set to 0. What does this indicate? empezar lección
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This is the last fragment (or the packet was not fragmented)
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What is the principle behind Path MTU Discovery using the DF flag? empezar lección
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Send packets with DF set; if a router cannot forward without fragmenting it returns an ICMP "fragmentation needed" message, so the sender lowers packet size
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What is the correct name for the PDU (Protocol Data Unit) at Layer 3 of the OSI model? empezar lección
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Why are both MAC addresses (Layer 2) and IP addresses (Layer 3) necessary? empezar lección
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MAC addresses are flat and non-hierarchical (routing is not possible); IP addresses are hierarchical and enable routing.
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How many bits does an IPv4 address consist of, and how many possible addresses does that theoretically yield? empezar lección
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32 bits, approximately 4.3 billion addresses (2^32)
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What is the decimal value of the binary number 10101100? empezar lección
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A host has the IP address 192.168.1.100 with a /24 subnet mask. What is the network portion and what is the host portion? empezar lección
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Network: 192.168.1, Host:
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What is the purpose of the 169.254.x. x (APIPA / Link-Local) address range? empezar lección
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It is automatically assigned when a device cannot reach a DHCP server.
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Which organization manages IP address allocation in Europe (including Austria)? empezar lección
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What was the main advantage of introducing CIDR in 1993 over classful addressing? empezar lección
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CIDR allows flexible prefix lengths (e.g., /22) instead of being restricted to fixed Class A, B, or C sizes, dramatically reducing address waste.
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Why are 2 addresses always subtracted from the total number of addresses in a subnet when calculating usable hosts? empezar lección
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Because the network address (all host bits = 0) and the broadcast address (all host bits = 1) cannot be assigned to any device.
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What is the broadcast address of the subnet 192.168.1.64/26? empezar lección
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What is VLSM (Variable Length Subnet Masking) and why is it used in practice? empezar lección
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VLSM allows different prefix lengths within the same address space, so subnets can be sized precisely for their actual needs (e.g., /25 for
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A company needs a subnet for exactly 500 hosts. What is the smallest prefix that meets this requirement? empezar lección
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How large is an IPv4 header without options, and what is the minimum value of the IHL field? empezar lección
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20 bytes; minimum IHL value = 5 (5 × 4 bytes = 20 bytes)
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Where does the reassembly of fragmented IPv4 packets take place? empezar lección
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Exclusively at the destination host — never at intermediate routers.
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What is the function of the default gateway for a host? empezar lección
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It is the IP address of the router interface on the local subnet; the host forwards all packets destined for other networks to this address.
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A routing table contains the entries 10.0.0.0/8, 10.50.0.0/16, and 0.0.0.0/0. A packet arrives destined for 10.50.5.1. Which route is used? empezar lección
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10.50.0.0/16, because /16 is the longest matching prefix.
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What happens at Layer 2 when a router receives a packet on interface eth0 and forwards it out interface eth1? empezar lección
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The router strips the old Layer 2 header (with the incoming MAC addresses), creates a new Layer 2 header with the appropriate MAC addresses for the next link, and decrements the TTL.
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A user reports no internet connectivity. Running ipconfig / ip addr shows the IP address 169.254.15.23. What is the most likely cause? empezar lección
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The device failed to reach a DHCP server and automatically assigned itself an APIPA address (169.254.x.x).
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How many bits does an IPv6 address use? empezar lección
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In which year was the global IANA IPv4 address pool fully depleted? empezar lección
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How many addresses does the IPv6 address space provide (approximately)? empezar lección
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What is the standard subnet prefix length for an IPv6 subnet? empezar lección
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IPv6 has no broadcast. What mechanism replaces broadcast functionality? empezar lección
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Which IPv6 address type is automatically generated on every interface, never routed beyond the local link, and starts with fe80:? empezar lección
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What is the prefix for IPv6 Global Unicast Addresses (GUA)? empezar lección
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What is the IPv6 loopback address? empezar lección
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The IPv6 header has a fixed size of how many bytes? empezar lección
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How many fields does the IPv6 header contain? empezar lección
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IPv6 does not include a header checksum. empezar lección
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In IPv6, routers can fragment packets if they are too large for a link. empezar lección
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The IPv6 "Hop Limit" field serves the same purpose as the IPv4 TTL field. empezar lección
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Expand the abbreviated IPv6 address 2001: db8:1 to its full form. Which is correct? empezar lección
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2001:0db8:0000:0000:0000:0000:0000:0001
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Why may the: (double colon) appear only once in an IPv6 address? empezar lección
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Using it twice would make the number of replaced zero groups ambiguous
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The IPv6 default route covering all destinations is expressed as: empezar lección
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In practice, which sub-range of the ULA block is actually used because the L bit is set to 1 for locally assigned prefixes? empezar lección
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Which prefix identifies all IPv6 multicast addresses? empezar lección
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The solicited-node multicast address used by Neighbor Discovery is formed from which prefix? empezar lección
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A GUA (Global Unicast) address typically divides into which three parts? empezar lección
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Global routing prefix, subnet ID, interface ID
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Within a /48 site allocation, how many bits remain for the subnet ID before the /64 interface boundary? empezar lección
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When pinging a link-local address such as fe80:1, why must the interface (zone ID) be specified, e.g. fe80:1%eth0? empezar lección
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Link-local addresses are not unique across interfaces, so the host must know which link to use
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Which of these addresses is a link-local address? empezar lección
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fe80: a1: b2ff: fe03: c4d5
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Roughly how many addresses does the 128-bit IPv6 space contain? empezar lección
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Which IPv6 header field is used for QoS / DSCP marking, analogous to the IPv4 Type of Service? empezar lección
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Which IPv6 header field identifies the type of the next header (upper-layer protocol or extension header)? empezar lección
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In the IPv6 Next Header field, which value indicates that UDP follows? empezar lección
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Optional IPv6 functionality (fragmentation, source routing, IPsec) is carried in what? empezar lección
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Extension headers chained via the Next Header field
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Which Next Header value identifies the Fragment extension header? empezar lección
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Because IPv6 routers never fragment, what must the source use to avoid oversized packets? empezar lección
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Which RFC defines stateless address autoconfiguration (SLAAC)? empezar lección
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During Duplicate Address Detection, what message does the host send and to which destination? empezar lección
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A Neighbor Solicitation to the solicited-node multicast address
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After inserting FFFE, the EUI-64 process flips which bit of the first byte? empezar lección
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The 7th bit (the U/L, universal/local bit)
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What privacy problem do Privacy Extensions (RFC 4941) solve? empezar lección
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EUI-64 embeds the MAC address, allowing devices to be tracked across networks
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In stateless DHCPv6 (RA flags A=1, O=1), how is the address obtained versus the extra configuration? empezar lección
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The address comes from SLAAC; DHCPv6 supplies extra info such as DNS servers
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On which UDP port does a DHCPv6 client listen? empezar lección
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Which RFC defines the Neighbor Discovery Protocol (NDP)? empezar lección
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Which ICMPv6 type number is assigned to the Neighbor Solicitation message? empezar lección
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Which NDP message lets a router inform a host of a better next-hop for a destination? empezar lección
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What does NAT64 accomplish? empezar lección
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It translates between IPv6 and IPv4 so IPv6-only clients can reach IPv4 servers
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Which compressed address is INVALID because it uses: more than once? empezar lección
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Which everyday IPv6 functions rely on link-local addresses? empezar lección
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NDP, SLAAC router solicitation/advertisement, and routing-protocol adjacencies
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IPv6 routers along the path are allowed to fragment packets that exceed the link MTU. empezar lección
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DHCPv6 servers listen on UDP port 546 while clients use 547. empezar lección
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IPv6 still includes a header checksum that routers recalculate at each hop. empezar lección
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The IPv6 loopback address: 1 is 64 bits long. empezar lección
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Neighbor Solicitation messages are sent to the solicited-node multicast address during address resolution and DAD. empezar lección
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When abbreviating an IPv6 address, the rule is that: (double colon) can be used to replace the longest consecutive run of all-zero groups. How many times can: appear in a single address? empezar lección
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In SLAAC, a host sends a Router Solicitation to which IPv6 multicast address? empezar lección
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DHCPv6 uses which UDP ports? empezar lección
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Privacy Extensions (RFC 4941) generate a random Interface ID to prevent empezar lección
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tracking of users across networks via their MAC-derived address
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What is the difference between forwarding (data plane) and routing (control plane)? empezar lección
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Forwarding moves individual packets based on the routing table; routing builds and maintains the routing table using protocols
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A routing table contains entries for 10.0.0.0/8, 10.1.0.0/16, and 10.1.1.0/24. A packet is destined for 10.1.1.50. Which entry will the router use? empezar lección
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10.1.1.0/24 (most specific match)
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To enable IPv4 forwarding on a Linux system, which file must contain the value 1? empezar lección
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/proc/sys/net/ipv4/ip_forward
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Static routes added with "ip route add" on Linux are lost after a reboot. empezar lección
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What is an Autonomous System (AS)? empezar lección
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A network or group of networks under one administrative authority with a unified routing policy
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Which algorithm does OSPF use to calculate the shortest path? empezar lección
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Dijkstra SPF (Shortest Path First)
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Which routing protocol uses the DUAL algorithm and supports unequal-cost load balancing? empezar lección
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BGP prevents routing loops by using the empezar lección
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AS-PATH attribute (a route that already contains the local AS is rejected)
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PAT (Port Address Translation) allows many private IP addresses to share a single public IP address. What field is used to distinguish the individual connections? empezar lección
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CGNAT (Carrier-Grade NAT) uses which reserved address range? empezar lección
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In a firewall rule set, the principle that "what is not explicitly permitted is denied" is called: empezar lección
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Which plane of operation is responsible for moving an individual packet from an input interface to an output interface? empezar lección
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The data plane (forwarding)
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A Linux route shows "proto kernel scope link src 192.168.1.10". What kind of route is this? empezar lección
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A directly connected route, auto-created when the IP address was assigned to the interface
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A routing table has 192.168.0.0/16 and 192.168.4.0/24. A packet for 192.168.9.7 arrives. Which entry is used? empezar lección
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192.168.0.0/16, because 192.168.9.7 does not fall inside 192.168.4.0/24
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Which is a genuine disadvantage of static routing? empezar lección
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It does not scale and provides no automatic failover when topology changes
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By default, a Linux machine with two NICs in different subnets will NOT route packets between them because empezar lección
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IP forwarding is disabled by default and the packet is silently dropped
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A router is a Layer 3 device that empezar lección
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connects different subnets and forwards packets between them based on IP addresses
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Which categorisation is correct? empezar lección
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OSPF and IS-IS are link-state IGPs; RIP and EIGRP are distance-vector IGPs; BGP is a path-vector EGP
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Which RIP loop-prevention technique stops a router from advertising a route back out the interface on which it was learned? empezar lección
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Which transport protocol and port does RIP use to exchange updates? empezar lección
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How does OSPF differ from RIP regarding update behaviour? empezar lección
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OSPF sends updates only when the topology changes, not periodically like RIP
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An OSPF Area Border Router (ABR) empezar lección
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connects one or more non-backbone areas to the backbone Area 0
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Which protocol uses a composite metric based primarily on bandwidth and delay and guarantees loop-free paths with DUAL? empezar lección
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Which statement about BGP route selection is most accurate? empezar lección
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BGP selects routes based on policy and business relationships, not strictly the shortest path
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What is the Administrative Distance of iBGP? empezar lección
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Two routing sources advertise the same network: OSPF (AD 110) and EIGRP internal (AD 90). Which route is installed in the routing table? empezar lección
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The EIGRP route, because a lower Administrative Distance is more trusted
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Which statement about Static NAT is correct? empezar lección
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It provides a permanent one-to-one mapping between a private and a public IP, allowing inbound access (e.g., to a server)
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Approximately how many simultaneous sessions can a single public IP support under PAT, limited by the source-port field? empezar lección
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Around 65,000 (limited by the 16-bit port range)
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A major drawback of NAT for certain applications is that it empezar lección
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breaks true end-to-end connectivity, complicating protocols like FTP, SIP/VoIP, and IPsec AH
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What is a key consequence of an ISP deploying CGNAT? empezar lección
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It adds a second layer of NAT, so customers share public IPs and inbound connections become harder
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Which conntrack state describes the very first packet of a new connection? empezar lección
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The default-deny (implicit deny) principle means empezar lección
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anything not explicitly permitted by a rule is dropped
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In iptables, packets generated by the local host itself traverse which chain? empezar lección
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Which Linux firewalling framework is the modern successor to iptables, unifying IPv4, IPv6, ARP, and bridge filtering? empezar lección
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Traceroute reveals the path a packet takes by empezar lección
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sending packets with incrementing TTL values and collecting the ICMP Time Exceeded replies from each hop
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In a TCP segment, what does the Acknowledgment Number field actually contain? empezar lección
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The sequence number of the NEXT byte the receiver expects to receive
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What is the semantic difference between the PSH and URG flags in TCP? empezar lección
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PSH tells the receiver to deliver buffered data to the application immediately; URG marks a region of the stream (via the Urgent Pointer) as priority out-of-band data
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What is the purpose of TCP keepalive probes? empezar lección
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To detect dead peers and keep idle-connection state alive by periodically probing a connection that has had no data for a long time
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What problem does Karn's algorithm solve in TCP's RTT estimation? empezar lección
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It excludes RTT samples from retransmitted segments, removing the ambiguity of not knowing whether an ACK matches the original or the retransmission
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Why is QUIC 0-RTT data considered vulnerable to replay attacks? empezar lección
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The early data is sent before the full handshake completes using a pre-shared key, so an attacker can capture and resend it; therefore only idempotent requests should be sent as 0-RTT
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How many bits is a port number? empezar lección
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What is the port range for well-known (system) ports? empezar lección
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What is the port range for dynamic/ephemeral ports? empezar lección
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How many values are used to uniquely identify a TCP connection (the so-called 4-tuple)? empezar lección
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Source IP + source port + destination IP + destination port (4 values)
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A web server can handle thousands of simultaneous TCP connections to the same port 443 because: empezar lección
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Each connection has a unique 4-tuple (different source IP and/or source port)
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Which "ss" command shows all listening TCP and UDP ports with their owning process? empezar lección
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Which well-known port does SSH use? empezar lección
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Which well-known port does HTTPS use? empezar lección
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Which well-known ports does FTP use for control and data? empezar lección
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21 (control) and 20 (data)
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Which well-known port does SMTP Submission (authenticated mail submission) use? empezar lección
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Which well-known port does POP3 (unencrypted) use? empezar lección
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Which well-known port does IMAP (unencrypted) use? empezar lección
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Which well-known port and protocol does DNS primarily use for queries? empezar lección
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Which well-known port does NTP (Network Time Protocol) use? empezar lección
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Which well-known port does SNMP Trap use to receive notifications? empezar lección
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Which well-known port does LDAP use? empezar lección
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Which well-known port does SMB (Windows file sharing) use? empezar lección
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Which registered port does PostgreSQL use? empezar lección
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DNS uses TCP instead of UDP in which situation(s)? empezar lección
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Zone transfers and responses larger than
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What is the minimum size of a TCP header? empezar lección
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Which IP protocol number is assigned to TCP? empezar lección
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Why does TCP use a 3-Way Handshake instead of a 2-Way Handshake? empezar lección
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Both sides need to synchronise independent Initial Sequence Numbers and confirm bidirectional reachability
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Why is the Initial Sequence Number (ISN) chosen pseudo-randomly? empezar lección
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To prevent TCP sequence prediction attacks (RFC 6528)
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How long does the TIME_WAIT state typically last? empezar lección
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Which TCP flag is used to abruptly terminate a connection (no graceful teardown)? empezar lección
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How does TCP detect packet loss using duplicate ACKs (Fast Retransmit)? empezar lección
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After receiving 3 duplicate ACKs (4 ACKs with the same number), retransmit the missing segment immediately
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A TCP connection in ESTABLISHED state means: empezar lección
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The 3-Way Handshake is complete and data can flow in both directions
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Many connections in SYN_RECEIVED state on a server most likely indicate: empezar lección
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A potential SYN flood attack
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What is the size of the UDP header? empezar lección
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How many fields does the UDP header contain? empezar lección
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4 (source port, destination port, length, checksum)
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Which of the following is NOT a typical reason to choose UDP over TCP? empezar lección
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Guaranteed in-order delivery
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Which statement about TCP and broadcast/multicast is correct? empezar lección
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TCP cannot perform broadcast or multicast — it is strictly point-to-point
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What does Flow Control protect against, and what mechanism is used? empezar lección
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Protects the receiver from buffer overflow; uses the Receiver Window (rwnd)
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During TCP Slow Start, how does the congestion window (cwnd) grow? empezar lección
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Exponentially — it doubles every RTT (starting from 1 MSS)
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When TCP detects loss via RTO timeout (no ACKs at all), how does it react? empezar lección
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Resets cwnd to 1 MSS and restarts Slow Start
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ECN (Explicit Congestion Notification) allows congestion to be signalled: empezar lección
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By routers marking IP packets instead of dropping them
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Which transport protocol does HTTP/3 use? empezar lección
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A "connection refused" error when connecting to a TCP service usually means: empezar lección
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No process is listening on the requested port (the server sent RST)
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Which sysctl tweak helps when a server runs out of ephemeral source ports? empezar lección
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Extending the ephemeral port range via net. ipv4. ip_local_port_range
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Which HTTP status code is returned when a new resource has been successfully created, for example after a successful POST? empezar lección
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What is the key difference between a 307 Temporary Redirect and a classic 302 Found? empezar lección
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307 guarantees the HTTP method (e.g. POST) is preserved on the redirected request, whereas
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Which status code tells the client that the resource was deliberately and permanently removed, and will not return (stronger than 404)? empezar lección
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A server is temporarily overloaded or down for maintenance and cannot handle the request right now. Which status code is correct? empezar lección
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A response includes the header "ETag: "v17-cd23"". On a later request, which header does the client send to ask the server whether the cached copy is still valid? empezar lección
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What does "Transfer-Encoding: chunked" allow a server to do? empezar lección
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Send the response body in pieces without knowing the total length in advance, omitting Content-Length
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What is the primary purpose of the HTTP OPTIONS method? empezar lección
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To discover the communication options (e.g. allowed methods) available for a resource, also used in CORS preflight
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During TLS negotiation, which extension lets the client and server agree on the application protocol (e.g. h2 for HTTP/2) within the handshake? empezar lección
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ALPN (Application-Layer Protocol Negotiation)
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An SRV record encodes which information for locating a service? empezar lección
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Priority, weight, port and target hostname for a named service
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What is "negative caching" in DNS? empezar lección
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Caching the fact that a name does NOT exist (an NXDOMAIN response) for a period defined by the SOA, to avoid repeated failing lookups
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Which of the following is NOT a Layer 7 protocol? empezar lección
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When you type "amazon. de" and press Enter, which step happens FIRST? empezar lección
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DNS resolves the name to an IP address
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HTTP is fundamentally based on which communication pattern? empezar lección
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In an HTTP request, which line comes first? empezar lección
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The request line (method + path + version)
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Which HTTP header tells the server which website the request is for, when multiple sites share one IP? empezar lección
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Which HTTP method is used to READ a resource without modifying it? empezar lección
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Which HTTP method is NEITHER safe NOR idempotent? empezar lección
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An HTTP status code starting with 2xx indicates: empezar lección
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An HTTP status code starting with 4xx indicates: empezar lección
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The client made a mistake
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Which status code means "the resource has not been modified, use your cached copy"? empezar lección
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What is the precise meaning of HTTP status code 403? empezar lección
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Authenticated, but you are not authorised to access this resource
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Which status code is typically returned by a reverse proxy when it cannot reach the backend? empezar lección
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What is the main performance limitation of HTTP/1.1 that browsers worked around by opening 6 to 8 parallel TCP connections? empezar lección
|
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Only one request can be in flight per TCP connection at a time
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Which feature of QUIC lets an HTTP/3 session survive a Wi-Fi to cellular network switch? empezar lección
|
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Connection migration (via connection IDs instead of the 4-tuple)
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HTTPS is best described as: empezar lección
|
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HTTP wrapped in a TLS encryption layer
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What is the primary purpose of HTTP cookies? empezar lección
|
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To give HTTP state — for example, to remember that a user is logged in
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Which architectural model does nginx primarily use, that makes it efficient for handling many idle connections? empezar lección
|
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Event-driven with a small number of worker processes
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Which API style is the default choice for public APIs and uses JSON over HTTP? empezar lección
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What is the primary function of DNS? empezar lección
|
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Translate human-readable domain names into IP addresses (and vice versa)
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Which symbol represents the DNS root zone? empezar lección
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When your browser needs to resolve a domain name, which server does it talk to directly? empezar lección
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Its configured resolver (often the ISP or a public resolver like 1.1.1.1)
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Which DNS record type maps a name to an IPv4 address? empezar lección
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Which DNS record type defines an alias from one name to another name? empezar lección
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Which DNS record type lists the authoritative nameservers for a domain? empezar lección
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Which DNS record type controls which Certificate Authorities are allowed to issue TLS certificates for a domain? empezar lección
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What does DNSSEC protect against? empezar lección
|
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Tampering and cache poisoning of DNS answers (integrity)
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What is one practical advantage of DoH over DoT? empezar lección
|
|
DoH traffic looks identical to normal HTTPS, making it much harder to block or distinguish on the network
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What is the primary defence against DNS cache poisoning attacks? empezar lección
|
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DNSSEC, combined with randomised source ports and transaction IDs
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A standard MTA from 1985 receives a modern MIME message with base64 attachments it cannot interpret. What does it do? empezar lección
|
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It relays the message unchanged, because MIME lives entirely inside the body and is invisible to SMTP
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Why is the LMTP / local pipe used by the MDA (typically port 24) distinct from SMTP relay on port 25? empezar lección
|
|
LMTP delivers to the final mailbox store and returns a separate status per recipient instead of queuing
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Which SMTP reply code is returned by QUIT when the server closes the session politely? empezar lección
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What is the difference between reply codes 450 and 550 for an unavailable mailbox? empezar lección
|
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450 is a transient failure (retry later);
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The SMTP VRFY command is usually disabled on public servers. Why? empezar lección
|
|
It lets an attacker confirm which addresses exist, leaking valid recipients for spam or attacks
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What is the advantage of BDAT (CHUNKING, RFC 3030) over the classic DATA command? empezar lección
|
|
BDAT sends a declared byte count so no dot-stuffing or end-of-data scanning is needed
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A receiving MTA does not advertise SMTPUTF8 but a sender tries to use an internationalised (UTF-8) address. What is the correct rejection? empezar lección
|
|
550 5.6.7 Need SMTPUTF8 support
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A DSN message is structured using which MIME content type defined by RFC 3464? empezar lección
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What is the primary purpose of the Message-ID: header? empezar lección
|
|
A globally unique, domain-scoped identifier used for threading (In-Reply-To/References) and loop/duplicate detection
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Which MIME multipart subtype glues independent parts such as a text body plus separate file attachments? empezar lección
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Quoted-printable encoding represents the byte 0xE2 (part of a UTF-8 sequence) on the wire as empezar lección
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The internationalised domain münchen. de is stored on the wire (without SMTPUTF8) as which Punycode form? empezar lección
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In a DKIM-Signature header, what does the bh= tag contain? empezar lección
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The hash of the canonicalised message body
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In a DKIM-Signature, "c=relaxed/relaxed" specifies empezar lección
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the canonicalisation algorithms applied to the header and body before hashing
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In a DMARC record, what does the pct= tag control? empezar lección
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The percentage of mail to which the published policy is applied (useful during rollout)
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What problem does ARC (Authenticated Received Chain, RFC 8617) solve? empezar lección
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It preserves original SPF/DKIM/DMARC results across forwarders so legitimate forwarded mail is not failed
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Which acronym describes the email client a user actually interacts with (Outlook, Thunderbird, Apple Mail)? empezar lección
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Which agent relays mail between domains using DNS MX lookups? empezar lección
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MTA — Mail Transfer Agent
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What does it mean that email is "store-and-forward"? empezar lección
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Each MTA accepts the whole message into a queue and only then attempts the next hop
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Which addresses constitute the SMTP "envelope" of an email? empezar lección
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Which Content-Transfer-Encoding is best suited for arbitrary binary attachments like PDFs and images? empezar lección
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Which SMTP port is reserved for MTA-to-MTA relay between domains? empezar lección
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Which SMTP port provides implicit TLS (encrypted from byte 1) for client submission, re-blessed by RFC 8314? empezar lección
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SMTP is best characterised as which kind of protocol? empezar lección
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Stateful, plain-text, line-oriented, command/response over a single TCP connection
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An SMTP reply code starting with 5xx means: empezar lección
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Permanent negative — stop, do not retry
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Which SMTP command requests the extended capability list (ESMTP)? empezar lección
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How does the DATA portion of an SMTP message end? empezar lección
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A line containing only a single dot (CRLF. CRLF)
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Which SMTP AUTH mechanism is considered modern and resistant to replay attacks? empezar lección
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When picking among multiple MX records for a domain, which one does the sender try first? empezar lección
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The MX record with the lowest preference number
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Which port is used for POP3 with implicit TLS (POP3S)? empezar lección
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Why is POP3 mostly considered legacy in 2026? empezar lección
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It cannot natively synchronise folders, flags or read-state across multiple devices
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Which port is used for IMAP with implicit TLS (IMAPS)? empezar lección
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What is the difference between FETCH BODY[TEXT] and FETCH BODY. PEEK[TEXT]? empezar lección
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BODY[TEXT] implicitly sets the Seen flag; BODY. PEEK[TEXT] retrieves the content without changing any flag
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How does a STRIPTLS downgrade attack work against mail submission? empezar lección
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An on-path attacker silently removes the STARTTLS line from the server's EHLO reply, causing the client to fall back to plaintext
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What does SPF (RFC 7208) publish? empezar lección
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A list of IP addresses authorised to send mail for a domain (as a DNS TXT record)
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Which DMARC policy value tells receivers to monitor only and apply no enforcement? empezar lección
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Which mechanism preserves SPF, DKIM and DMARC results across mailing-list forwarders that would otherwise break them? empezar lección
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[moodle]ARC — Authenticated Received Chain (RFC 8617)
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How many layers does the OSI reference model define? empezar lección
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Which OSI layer is closest to the end user? empezar lección
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What is the Protocol Data Unit (PDU) at the OSI Transport layer? empezar lección
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What is the Protocol Data Unit (PDU) at the OSI Data Link layer? empezar lección
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Which is the correct order of the OSI layers from Layer 1 to Layer 7? empezar lección
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Physical, Data Link, Network, Transport, Session, Presentation, Application
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Which OSI layer provides end-to-end data delivery, reliability, and flow control? empezar lección
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Transport layer (Layer 4)
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Which OSI layer transmits raw bits over a physical medium? empezar lección
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Which OSI layer handles data format translation, encryption, and compression? empezar lección
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Presentation layer (Layer 6)
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Character encoding such as ASCII and Unicode is a task of which OSI layer? empezar lección
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Presentation layer (Layer 6)
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Data compression to reduce the size of transmitted data is performed at which OSI layer? empezar lección
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Presentation layer (Layer 6)
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Dialog control and the management of full-duplex, half-duplex, and simplex communication modes is a function of which OSI layer? empezar lección
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Which of the following are listed as examples of Session layer mechanisms? empezar lección
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NetBIOS, RPC, and login sessions
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Which of the following is an Application layer protocol used for name resolution? empezar lección
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SMTP, POP3, and IMAP are Application layer protocols used for which service? empezar lección
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Which Transport layer protocol is connection-oriented and provides reliable delivery? empezar lección
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Which characteristic best describes UDP compared to TCP? empezar lección
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Low overhead and fast, with no guaranteed delivery
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What identifies the application associated with a Transport layer communication? empezar lección
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At the Network layer, what kind of address identifies a device across networks? empezar lección
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IP address (logical address)
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Which protocols operate at the OSI Network layer? empezar lección
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Packet fragmentation, when needed, is handled by which OSI layer? empezar lección
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How does the Data Link layer detect errors in received frames? empezar lección
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Using a Cyclic Redundancy Check (CRC)
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Which protocols are associated with the Data Link layer? empezar lección
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Ethernet (802.3) and Wi-Fi (802.11)
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Which of the following does the Physical layer define? empezar lección
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Electrical, optical, or radio signals, voltage levels, and signal timing
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In the OSI model, the upper layers (5, 6, and 7) are often grouped as which category? empezar lección
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How many layers does the TCP/IP model commonly have? empezar lección
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4 (sometimes described as 5)
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The TCP/IP Internet layer corresponds to which OSI layer? empezar lección
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The TCP/IP Transport layer corresponds to which OSI layer? empezar lección
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Transport layer (Layer 4)
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Which protocols are listed for the TCP/IP Internet layer? empezar lección
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Which organization is credited with developing the TCP/IP model? empezar lección
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In practice, why do engineers still use OSI layer numbers (such as "Layer 3 switch")? empezar lección
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OSI numbers are used to describe concepts, while TCP/IP protocols are used for actual communication
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During encapsulation, in what order is the data transformed from the Application layer down to the Physical layer? empezar lección
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Data, Segment, Packet, Frame, Bits
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What part of a PDU carries the actual user information rather than control information? empezar lección
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According to the slides, what does PDU stand for? empezar lección
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What does PCI (Protocol Control Information) refer to in a layered model? empezar lección
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The header and/or trailer added by each layer
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What is a Service Access Point (SAP)? empezar lección
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The interface between adjacent layers
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Which device operates at the Physical layer and simply regenerates or repeats signals to all ports? empezar lección
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A router makes forwarding decisions based on which type of address? empezar lección
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A firewall that filters traffic based on IP addresses and TCP/UDP port numbers operates at which OSI layers? empezar lección
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Network and Transport layers (Layers 3 and 4)
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Which organization manages Internet standards published as RFCs? empezar lección
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IETF (Internet Engineering Task Force)
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Which standards organization is part of the United Nations and focuses on telecommunication standards? empezar lección
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ITU (International Telecommunication Union)
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Which RFC defines TCP (Transmission Control Protocol)? empezar lección
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Which RFC number corresponds to the very first RFC, "Host Software"? empezar lección
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What is the definition of a protocol given in the slides? empezar lección
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The rules for the format, order, and actions of messages exchanged between communicating entities
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What is the main reason for using a layered architecture in networking? empezar lección
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Complexity management — breaking a huge problem into smaller, manageable parts
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Which benefit of layering means each layer only needs to know the interface of adjacent layers? empezar lección
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A LAN speed is given as "1-10 Gbps" in the slides. What does Gbps measure? empezar lección
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Data rate (gigabits per second)
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Bit rate, voltage levels, and signal timing are defined at which OSI layer? empezar lección
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The Data Link layer adds both a header and a trailer to data from Layer 3. What is the trailer field used for error detection commonly called? empezar lección
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Frame Check Sequence (FCS)
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During encapsulation, which layer adds the header that turns "Data" into a "Segment"? empezar lección
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The Transport layer (Layer 4)
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A device described as operating at "Layer 3" forwards data based on which information? empezar lección
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When two hosts negotiate, maintain, and gracefully end a logical conversation independent of the underlying transport connection, which OSI layer is conceptually responsible? empezar lección
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In the TCP/IP model, the Session, Presentation, and Application layers of OSI are combined into a single Application layer. empezar lección
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