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Computer networks interview questions

Computer networks interview questions for freshers cover the OSI and TCP/IP models, TCP versus UDP, IP addressing and subnetting, devices such as routers and switches, and protocols like DNS, DHCP, HTTP and ARP. A favourite closing question is what happens when you type a website address into a browser, so practise explaining it step by step.

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  • 30 questions with answers

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Short answers for IT services, support and networking rounds, from the models to the classic scenario questions.

Questions with answers30in 5 groups

Basic computer networks interview questions for freshers

5 questions

  1. What is a computer network?

    Basic

    Two or more devices connected so they can share data and resources such as files, printers and an internet connection, using agreed rules called protocols.

  2. What is the difference between LAN, MAN and WAN?

    Basic

    A LAN (local area network) covers a small area such as an office or a college lab. A MAN (metropolitan area network) covers a city. A WAN (wide area network) connects networks across countries; the internet is the largest WAN.

  3. What are the common network topologies?

    Basic

    Bus (all devices on one cable), star (all devices connected to a central switch, the most common today), ring (each device connected to two neighbours), mesh (devices connected to many others for redundancy) and hybrid (a mix).

  4. What is the difference between unicast, multicast and broadcast?

    Basic

    Unicast sends data to one device, multicast to a chosen group of devices, and broadcast to every device on the local network.

  5. What is bandwidth, and how is it different from latency?

    Basic

    Bandwidth is how much data a link can carry per second, such as 100 Mbps. Latency is how long data takes to travel from one end to the other, in milliseconds. A link can have high bandwidth and still feel slow if latency is high.

OSI and TCP/IP model questions

4 questions

  1. What are the seven layers of the OSI model?

    Basic

    From bottom to top: - Physical: bits on cables and radio. - Data link: frames and MAC addresses (switches). - Network: packets and IP addresses (routers). - Transport: end-to-end delivery with TCP or UDP and ports. - Session: opening and closing sessions. - Presentation: formats, encryption and compression. - Application: what apps use, such as HTTP, DNS and SMTP.

  2. What are the layers of the TCP/IP model?

    Basic

    Four layers: network access (or link), internet (IP), transport (TCP and UDP) and application (HTTP, DNS, FTP and others). It is the model the internet actually uses; OSI is mostly used to teach and to troubleshoot.

  3. At which layer do a hub, a switch and a router work?

    Basic

    A hub works at the physical layer, a switch at the data link layer (it uses MAC addresses) and a router at the network layer (it uses IP addresses). Layer 3 switches can also route.

  4. What is encapsulation?

    Intermediate

    As data moves down the layers, each layer adds its own header: the transport layer makes a segment, the network layer a packet and the data link layer a frame. The receiver removes the headers in reverse order.

TCP, UDP and protocol questions

8 questions

  1. What is the difference between TCP and UDP?

    Basic

    TCP is connection-oriented: it sets up a connection, numbers the data, resends lost pieces and delivers everything in order, so it is reliable but slower. UDP sends datagrams with no connection or delivery guarantee, so it is faster. Email and most web pages (HTTP/1.1 and HTTP/2) use TCP; HTTP/3 runs over QUIC, which is built on UDP. Video calls, online games and DNS lookups often use UDP.

  2. What is the TCP three-way handshake?

    Basic

    How TCP opens a connection: the client sends SYN, the server replies SYN-ACK, and the client answers ACK. After these three messages both sides can send data.

  3. What does DNS do?

    Basic

    DNS (Domain Name System) translates names such as example.com into IP addresses that computers use. Your device asks a DNS resolver, which looks up the answer through root, top-level-domain and authoritative servers and caches it.

  4. What does DHCP do?

    Basic

    DHCP automatically gives a device an IP address, subnet mask, default gateway and DNS server when it joins a network. The steps are Discover, Offer, Request and Acknowledge (DORA).

  5. What does ARP do?

    Intermediate

    ARP (Address Resolution Protocol) finds the MAC address that belongs to an IP address on the local network, by broadcasting "who has this IP?" and caching the reply.

  6. What is the difference between HTTP and HTTPS?

    Basic

    HTTPS is HTTP sent over an encrypted TLS connection, so data cannot be read or changed in transit, and the server proves its identity with a certificate. HTTP uses port 80 by default and HTTPS uses port 443.

  7. Which port numbers should you know?

    Basic

    FTP 20 and 21, SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67 and 68, HTTP 80, POP3 110, IMAP 143, HTTPS 443 and RDP 3389.

  8. What does ping use, and what does traceroute show?

    Intermediate

    ping sends ICMP echo requests to check whether a host is reachable and how long replies take. traceroute (tracert on Windows) shows each router hop on the path to the destination, which helps find where a connection slows down or fails.

IP addressing and subnetting questions

7 questions

  1. What is the difference between IPv4 and IPv6?

    Basic

    IPv4 addresses are 32 bits, written as four numbers such as 192.168.1.10, giving about 4.3 billion addresses. IPv6 addresses are 128 bits, written in hexadecimal groups separated by colons, giving vastly more addresses, plus simpler headers and built-in support for autoconfiguration.

  2. What is the difference between an IP address and a MAC address?

    Basic

    An IP address is a logical address that can change and is used to route data between networks. A MAC address is a 48-bit hardware address set on the network card and used for delivery inside the local network.

  3. What are private IP addresses?

    Basic

    Address ranges reserved for internal networks and not routed on the internet: 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255 and 192.168.0.0 to 192.168.255.255. 127.0.0.1 is the loopback address, meaning "this computer".

  4. What is a subnet mask?

    Intermediate

    It splits an IP address into the network part and the host part. With 255.255.255.0 (written /24), the first three numbers are the network and the last is the host, giving 256 addresses, of which 254 can be used by devices.

  5. How many usable hosts does a /26 network have?

    Intermediate

    A /26 leaves 32 - 26 = 6 host bits, so 2 to the power 6 = 64 addresses. Subtract the network and broadcast addresses: 62 usable hosts.

  6. What is a default gateway?

    Basic

    The router a device sends traffic to when the destination is outside its own network. On most home networks it is the Wi-Fi router's address.

  7. What is NAT?

    Intermediate

    Network Address Translation lets many devices with private IP addresses share one public IP address. The router rewrites the addresses on outgoing traffic and keeps a table to send replies back to the right device.

Devices, security and scenario questions

6 questions

  1. What is the difference between a hub, a switch and a router?

    Basic

    A hub repeats every signal to all ports. A switch learns which MAC address is on which port and sends frames only where they need to go. A router connects different networks and forwards packets between them using IP addresses.

  2. What is a VLAN?

    Intermediate

    A virtual LAN splits one physical switch network into separate logical networks, such as one for staff and one for guests. Devices in different VLANs need a router or layer 3 switch to talk to each other.

  3. What is a firewall?

    Basic

    A hardware device or software that allows or blocks traffic according to rules, such as source and destination addresses, ports and protocols, to protect a network from unwanted access.

  4. What is a VPN?

    Basic

    A virtual private network creates an encrypted tunnel over the internet between a device and a network, so remote employees can use office systems securely as if they were on the office network.

  5. What happens when you type a website address into a browser?

    Intermediate
    • The browser checks its cache, then asks DNS for the site's IP address.
    • It opens a TCP connection to the server (three-way handshake), then a TLS handshake for HTTPS.
    • It sends an HTTP request for the page.
    • The server sends back a response with the HTML.
    • The browser loads the CSS, scripts and images the HTML asks for, and renders the page.

  6. A user says "the internet is not working". How do you troubleshoot?

    Intermediate

    Work up the layers. Check the cable or Wi-Fi connection and whether other users are affected. Check the IP settings (ipconfig or ip addr). Ping the default gateway, then a public IP address, then a domain name: if the IP works but the name does not, it is a DNS problem.

How to prepare for a computer networks interview

  1. Learn the OSI layers with one example protocol and one device for each, and be ready to say where TCP, IP, a switch and a router fit.

  2. Practise subnetting by hand: for /24 to /30, work out the number of addresses, usable hosts, network address and broadcast address.

  3. Memorise the common port numbers and what each protocol does: HTTP, HTTPS, DNS, DHCP, FTP, SSH and SMTP.

  4. Rehearse "what happens when you type a URL" aloud in under two minutes, covering DNS, TCP, TLS, HTTP and rendering.

  5. Try the commands yourself: ping, tracert or traceroute, ipconfig or ip addr, nslookup. Interviewers for support roles often ask what each output means.

Which jobs ask computer networks interview questions?

Network engineer, system administrator, IT support, cloud and DevOps roles ask them in depth. Software roles at service companies usually ask the basics: OSI layers, TCP versus UDP, DNS and what happens when you type a URL. Many of those technical rounds also include C basics; see our C interview questions.

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Questions freshers ask

The OSI layers, TCP versus UDP, the three-way handshake, IPv4 versus IPv6, IP versus MAC address, what DNS and DHCP do, subnet masks, common ports, and what happens when you type a URL in a browser.

Cisco's CCNA is the best-known entry-level networking certification, and CompTIA Network+ is a vendor-neutral option. Neither is needed for a software role, but either helps for network or support jobs.

Know the OSI and TCP/IP models, TCP versus UDP, IP addressing basics, DNS, HTTP and HTTPS, and be able to explain what happens when you load a web page. Subnetting and routing protocols matter more for networking roles.