Networking interview question · Most asked
What happens, at the network level, when you type a URL into a browser and press Enter?
Summary
The browser resolves the host name with DNS, opens a TCP connection (or QUIC over UDP for HTTP/3) to the server's IP and port 443, performs a TLS handshake that verifies the certificate, sends an HTTP request and renders the response, fetching further resources over the same connection.
Longer Answer
In more detail: the OS checks its cache and asks a recursive resolver, which may walk root, TLD and authoritative servers. The route to the server's IP goes via the default gateway and many routers; NAT may rewrite the source address. TCP's three-way handshake costs one round trip and TLS 1.3 another, which is why connection reuse, HTTP/2 multiplexing and CDNs near users matter. The response may come from a CDN edge cache rather than the origin.
Key Points
- DNS lookup, possibly cached at several levels.
- TCP handshake (or QUIC) to port 443.
- TLS handshake and certificate validation.
- HTTP request/response, then more requests on the same connection.
Deep dive: Networking Tutorial: Networking Introduction
Explanation
What interviewers look for
A clear, ordered walk through DNS, TCP, TLS and HTTP, with awareness of caching, round trips and where things can fail.
Follow-up questions
- Where would you look if this takes 3 seconds?
- What changes with HTTP/3?
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