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Computer Networks Interview Questions — Set 1

25 networking interview questions — OSI layers, TCP/UDP, IP addressing, subnetting, DNS/DHCP and the protocols behind every packet.

Computer Networks25 questionsInstant feedback + explanationsup to ≈150 XP

One question at a time — answer, learn from the explanation, and keep your combo alive. Stuck? You have two 50:50s and one hint (each halves that question's XP).

Practice sheet: all 25 questions

Prefer reading first? Every question from this practice set, with the answer and a worked explanation. (Solving them in the practice set above earns XP — up to ≈150.)

  1. Q1. How many layers does the OSI reference model have?

    • a. 4
    • b. 5
    • c. 6
    • d. 7
    Show answer & explanation

    Answer: D. 7

    OSI has 7 layers: Physical, Data Link, Network, Transport, Session, Presentation, Application (mnemonic: "Please Do Not Throw Sausage Pizza Away"). The practical TCP/IP model compresses these into 4 — interviews often ask you to map one onto the other.

  2. Q2. Routing between networks is the responsibility of which OSI layer?

    • a. Data link layer
    • b. Network layer
    • c. Transport layer
    • d. Session layer
    Show answer & explanation

    Answer: B. Network layer

    The network layer (layer 3) owns logical addressing (IP) and routing — choosing the path across networks. The data link layer only delivers frames within one network segment; transport handles end-to-end delivery between processes.

  3. Q3. Which statement about TCP and UDP is correct?

    • a. TCP is connectionless; UDP is connection-oriented
    • b. TCP is connection-oriented and reliable; UDP is connectionless and unreliable
    • c. Both guarantee in-order delivery
    • d. UDP retransmits lost packets automatically
    Show answer & explanation

    Answer: B. TCP is connection-oriented and reliable; UDP is connectionless and unreliable

    TCP sets up a connection, acknowledges data, retransmits losses and preserves order; UDP just sends datagrams — no handshake, no recovery, minimal overhead. That trade-off (reliability vs latency) decides which protocol an application uses.

  4. Q4. Which application is the BEST fit for UDP rather than TCP?

    • a. File download
    • b. Email delivery
    • c. Live video streaming
    • d. Bank transactions
    Show answer & explanation

    Answer: C. Live video streaming

    Live streaming prefers UDP: a lost frame is better skipped than retransmitted late, and the handshake/ACK overhead of TCP adds latency. Files, email and payments need every byte intact and ordered — that's TCP territory.

  5. Q5. What is the correct sequence of TCP's three-way handshake?

    • a. SYN → SYN-ACK → ACK
    • b. ACK → SYN → SYN-ACK
    • c. SYN → ACK → SYN-ACK
    • d. SYN-ACK → SYN → ACK
    Show answer & explanation

    Answer: A. SYN → SYN-ACK → ACK

    The client sends SYN with its initial sequence number, the server replies SYN-ACK (its own SYN plus an ACK of the client's), and the client's final ACK completes the connection. Both sides have now exchanged and confirmed sequence numbers.

  6. Q6. What is the default port number for HTTPS?

    • a. 21
    • b. 80
    • c. 443
    • d. 8080
    Show answer & explanation

    Answer: C. 443

    HTTPS uses port 443; plain HTTP uses 80. The other classics worth memorising: FTP 21, SSH 22, Telnet 23, SMTP 25, DNS 53, POP3 110, IMAP 143 — port questions are free marks in written rounds.

  7. Q7. What is the primary job of DNS?

    • a. Assigning IP addresses to new devices
    • b. Translating domain names into IP addresses
    • c. Encrypting web traffic
    • d. Routing packets between networks
    Show answer & explanation

    Answer: B. Translating domain names into IP addresses

    DNS resolves human-friendly names (placementclass.com) to IP addresses via a distributed hierarchy of name servers. Assigning addresses is DHCP's job — the two get swapped in exams because both "give" you an IP in casual speech.

  8. Q8. How many bits long is an IPv4 address?

    Show answer & explanation

    Answer: 32

    IPv4 addresses are 32 bits — four 8-bit octets written in dotted decimal (e.g. 192.168.1.1), giving about 4.3 billion addresses. Address exhaustion is why IPv6 moved to 128 bits.

  9. Q9. How many usable host addresses does a /26 subnet provide?

    • a. 64
    • b. 62
    • c. 32
    • d. 126
    Show answer & explanation

    Answer: B. 62

    /26 leaves 32 − 26 = 6 host bits → 2⁶ = 64 addresses, minus the network and broadcast addresses = 62 usable. The −2 is the whole question; 64 is the planted answer for anyone who forgets it.

  10. Q10. Which of these IPv4 addresses is a private address?

    • a. 8.8.8.8
    • b. 192.168.10.5
    • c. 172.32.0.1
    • d. 11.0.0.1
    Show answer & explanation

    Answer: B. 192.168.10.5

    The private (RFC 1918) ranges are 10.0.0.0/8, 172.16.0.0/12 (172.16–172.31 only) and 192.168.0.0/16. 192.168.10.5 fits; 172.32.x.x is just OUTSIDE the 172 block — that boundary is exactly what this question tests.

  11. Q11. A MAC address is how many bits long, and at which layer does it operate?

    • a. 32 bits, network layer
    • b. 48 bits, data link layer
    • c. 64 bits, physical layer
    • d. 128 bits, transport layer
    Show answer & explanation

    Answer: B. 48 bits, data link layer

    MAC addresses are 48 bits (six hex pairs like 3C:5A:B4:xx:xx:xx), burned into the NIC and used by the data link layer to deliver frames on a local segment. IPs route between networks; MACs deliver within one.

  12. Q12. The ARP protocol is used to:

    • a. Map an IP address to a MAC address
    • b. Map a MAC address to a hostname
    • c. Assign IP addresses dynamically
    • d. Encrypt frames on the LAN
    Show answer & explanation

    Answer: A. Map an IP address to a MAC address

    ARP broadcasts "who has this IP?" on the LAN and the owner replies with its MAC — the mapping is cached for future frames. Reverse lookups (MAC → IP) were RARP's job historically; DHCP is what assigns addresses.

  13. Q13. What is the key difference between a hub and a switch?

    • a. A hub forwards frames only to the destination port; a switch broadcasts to all ports
    • b. A switch learns MAC addresses and forwards frames only to the right port; a hub repeats to every port
    • c. They are identical in function
    • d. A hub operates at the network layer
    Show answer & explanation

    Answer: B. A switch learns MAC addresses and forwards frames only to the right port; a hub repeats to every port

    A hub is a layer-1 repeater — every frame goes to every port, one big collision domain. A switch (layer 2) learns which MAC lives on which port and forwards frames only there, giving each port its own collision domain.

  14. Q14. At which OSI layer does a router primarily operate?

    • a. Layer 1 (Physical)
    • b. Layer 2 (Data Link)
    • c. Layer 3 (Network)
    • d. Layer 4 (Transport)
    Show answer & explanation

    Answer: C. Layer 3 (Network)

    Routers make forwarding decisions on IP addresses — layer 3. The device ladder to remember: hub = layer 1, switch = layer 2, router = layer 3; each device reads one layer deeper into the packet.

  15. Q15. Which protocol automatically assigns IP addresses, subnet masks and gateways to hosts joining a network?

    • a. DNS
    • b. DHCP
    • c. ARP
    • d. ICMP
    Show answer & explanation

    Answer: B. DHCP

    DHCP leases network configuration via the DORA exchange — Discover, Offer, Request, Acknowledge. Without it every device would need manual IP configuration; with it, joining Wi-Fi "just works".

  16. Q16. HTTP is called a stateless protocol because:

    • a. It never uses TCP connections
    • b. The server retains no memory of previous requests from the same client
    • c. It cannot transfer dynamic pages
    • d. It works without IP addresses
    Show answer & explanation

    Answer: B. The server retains no memory of previous requests from the same client

    Each HTTP request stands alone — the server doesn't remember who you are between requests. Sessions, logins and carts are built on top with cookies/tokens that the client re-sends every time. That layering is the interview follow-up.

  17. Q17. TCP's sliding window mechanism provides:

    • a. Encryption
    • b. Flow control
    • c. Name resolution
    • d. Physical addressing
    Show answer & explanation

    Answer: B. Flow control

    The window tells the sender how much unacknowledged data it may have in flight, sized to what the receiver can buffer — that's flow control (protecting the receiver). Congestion control is the related-but-different mechanism protecting the network itself.

  18. Q18. In TCP congestion control, the slow-start phase:

    • a. Increases the congestion window exponentially until a threshold
    • b. Keeps the window constant forever
    • c. Decreases the window linearly
    • d. Disables acknowledgements
    Show answer & explanation

    Answer: A. Increases the congestion window exponentially until a threshold

    Despite the name, slow start doubles the congestion window every RTT (exponential growth) from a small initial value, until it hits the ssthresh threshold — then congestion avoidance takes over with linear growth. "Slow" refers only to the small starting point.

  19. Q19. CRC (Cyclic Redundancy Check) is primarily used for:

    • a. Error detection in transmitted frames
    • b. Encrypting frames
    • c. Compressing data
    • d. Assigning addresses
    Show answer & explanation

    Answer: A. Error detection in transmitted frames

    CRC treats the frame as a polynomial, divides by a generator polynomial, and appends the remainder; the receiver repeats the division and a non-zero remainder means corruption. It detects errors (especially bursts) — it does not correct them; correction needs codes like Hamming.

  20. Q20. In stop-and-wait ARQ, how many frames can be outstanding (sent but unacknowledged) at a time?

    • a. 1
    • b. 2
    • c. Window size
    • d. Unlimited
    Show answer & explanation

    Answer: A. 1

    Stop-and-wait sends one frame and idles until its ACK returns — exactly one outstanding frame. On long links this wastes almost all the bandwidth, which is precisely the problem sliding-window protocols (Go-Back-N, Selective Repeat) solve.

  21. Q21. The ping utility uses which protocol?

    • a. ICMP
    • b. SMTP
    • c. FTP
    • d. SNMP
    Show answer & explanation

    Answer: A. ICMP

    ping sends ICMP Echo Request packets and measures the Echo Replies — testing reachability and round-trip time. ICMP is IP's control/diagnostics companion protocol; it carries no application data.

  22. Q22. traceroute discovers the path to a destination by manipulating which IP header field?

    • a. Checksum
    • b. TTL (Time To Live)
    • c. Fragment offset
    • d. Source address
    Show answer & explanation

    Answer: B. TTL (Time To Live)

    traceroute sends probes with TTL = 1, 2, 3, …; each router that decrements TTL to zero drops the probe and replies with an ICMP "Time Exceeded", revealing itself. Hop by hop, the whole path emerges — an elegant abuse of a loop-prevention field.

  23. Q23. Which protocol is used to SEND email between mail servers?

    • a. POP3
    • b. IMAP
    • c. SMTP
    • d. HTTP
    Show answer & explanation

    Answer: C. SMTP

    SMTP pushes mail — client to server and server to server. POP3 and IMAP are retrieval protocols for reading your inbox (POP3 downloads-and-deletes, IMAP syncs across devices). Send vs receive is the whole distinction.

  24. Q24. Which of the following protocols operate at the application layer? (Select all that apply.)

    • a. HTTP
    • b. DNS
    • c. SMTP
    • d. TCP
    Show answer & explanation

    Answer: A. HTTP · B. DNS · C. SMTP

    HTTP, DNS and SMTP are application-layer protocols — they define message formats for user-facing services. TCP sits a layer below at transport, carrying all three. DNS is the tempting wrong-exclusion because it mostly rides on UDP, but it's still application layer.

  25. Q25. NAT (Network Address Translation) allows:

    • a. Multiple devices with private IPs to share one public IP
    • b. Encryption of all outgoing traffic
    • c. Direct MAC-to-MAC communication across the internet
    • d. Automatic domain name assignment
    Show answer & explanation

    Answer: A. Multiple devices with private IPs to share one public IP

    NAT rewrites private source addresses to the router's public IP (tracking each flow by port), letting a whole LAN share one public address — the main workaround for IPv4 scarcity. Your home Wi-Fi does this for every device on it.