Fifteen common networking interview questions
- Explain the OSI model and the TCP/IP model.
- TCP versus UDP — when would you use each?
- Describe the TCP three-way handshake.
- How does TCP provide reliability and flow control?
- How does DNS resolution work?
- What happens when you type a URL into a browser?
- HTTP versus HTTPS — what does TLS add?
- What are common HTTP methods and status codes?
- IPv4 versus IPv6?
- What is subnetting, and what does /24 mean?
- What do NAT and DHCP do?
- Router versus switch versus hub?
- What is ARP?
- What is a load balancer, and at which layers can it work?
- What is latency versus bandwidth?
Models and transport
- OSI (7 layers): physical, data link, network, transport, session, presentation, application. TCP/IP (4 layers): link, internet, transport, application — the model the internet actually uses.
- TCP: connection-oriented, reliable, ordered delivery with retransmission, flow control and congestion control — for web pages, email, file transfer.
- UDP: connectionless, no delivery guarantee, low overhead — for real-time voice and video, gaming and DNS queries, where late data is useless.
- Three-way handshake: client sends SYN, server replies SYN-ACK, client sends ACK; sequence numbers are agreed so data can be ordered and acknowledged.
What happens when you type a URL
- The browser parses the URL and checks its caches.
- DNS resolves the domain: browser and OS caches, then a recursive resolver queries root, top-level-domain and authoritative servers.
- The browser opens a TCP connection to the IP address (or QUIC over UDP for HTTP/3).
- A TLS handshake authenticates the server's certificate and agrees encryption keys.
- The browser sends an HTTP request; the server (often behind a load balancer and CDN) responds.
- The browser parses HTML, fetches CSS, JavaScript and images, builds the page and renders it.
Addressing and devices
| Term | Meaning |
|---|---|
| IPv4 / IPv6 | 32-bit addresses (about 4.3 billion) / 128-bit addresses, effectively unlimited |
| /24 subnet | First 24 bits are the network; 256 addresses, 254 usable hosts |
| NAT | Maps private addresses to a public one, letting many devices share an IP |
| DHCP | Assigns IP addresses and network settings automatically |
| ARP | Finds the MAC address for an IP address on the local network |
| Switch / router | Forwards frames within a network by MAC / routes packets between networks by IP |
HTTP, TLS and load balancing
HTTPS is HTTP over TLS, which provides encryption, integrity and server authentication via certificates. Know GET, POST, PUT, PATCH and DELETE and the status code families: 2xx success, 3xx redirection, 4xx client error, 5xx server error. Load balancers work at layer 4 (routing TCP or UDP connections by IP and port, fast) or layer 7 (routing HTTP requests by path, headers or cookies, more flexible). See REST API interview questions.
Latency and bandwidth
Latency is the time for data to travel from one point to another; bandwidth is how much data can move per second. A satellite link can have high bandwidth and terrible latency. For web performance, latency usually dominates because pages require many round trips — which is why CDNs, connection reuse and fewer requests help more than raw bandwidth.
Practising
Answer these out loud, in two minutes or less each, and expect a follow-up on every claim. A free AI interview assistant such as CoPilot Interview can run networking questions as a mock interview built from your résumé and score each answer. Rehearse the URL walkthrough until you can do it in two minutes and then zoom into any step. Network engineers should also see network engineer interview questions.
Disclosure: CoPilot Interview is our product, so weigh what we say about it accordingly. Everything else on this page applies whichever tool you use, or none.
Frequently asked questions
What is the difference between TCP and UDP?
TCP is connection-oriented with reliable, ordered delivery, retransmission and congestion control. UDP is connectionless with no delivery guarantees and lower overhead, suited to real-time traffic.
What happens when you type a URL into a browser?
The browser checks caches, resolves the domain via DNS, opens a TCP or QUIC connection, performs a TLS handshake, sends an HTTP request, receives the response and renders the page while fetching its resources.
What is the TCP three-way handshake?
The client sends SYN, the server replies SYN-ACK, and the client sends ACK, establishing a connection with agreed sequence numbers.
What does a /24 subnet mean?
The first 24 bits of the address identify the network, leaving 8 bits for hosts: 256 addresses, of which 254 are usable.
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