What Happens When a User Enters a URL in the Browser?

maang.io System Design Series

What is this?

You type https://www.example.com into the address bar, press Enter, and about a quarter of a second later a fully painted page is staring back at you. In that quarter of a second, a small miracle of coordination happened: a name became a number, two strangers on opposite sides of the planet agreed on a shared secret, a request crossed dozens of routers, bounced through caches and proxies and balancers, woke up a server, and came home — all before you could blink.

This chapter is the capstone of the whole 101 tier. Every other topic you've learned — DNS, TCP, TLS, HTTP, the OSI model, load balancers, CDNs, reverse proxies, REST — was a single instrument. Here we hear the whole orchestra play one piece. We're going to follow one click on its entire journey, like tracing a letter from the moment it leaves your hand, through the sorting offices and mail trucks, into your friend's mailbox, and all the way back with their reply.

The one-line idea: typing a URL kicks off an ordered relay — parse → resolve a name to an IP → open a connection → agree on encryption → send a request → route it through the edge to a server → get a response → paint it — and every box in that relay is one of the other chapters in this tier, doing its one job.

The order matters enormously, and it's where most people get tripped up. You can't open a TCP connection until DNS has handed you an IP. You can't do the TLS handshake until TCP is up. You can't send the HTTP request until TLS is done. Get the sequence right and the whole system makes sense. Let's walk it.