What Actually Happens When You Type a URL

Classic interview question. Everyone gets the basics. DNS, TCP, HTTP. Ask for details and they fall apart.

DNS: Finding the Server

You type google.com. Browser needs an IP address.

This takes 20-120ms. Or 0ms if cached.

Here's what people miss: DNS uses UDP, not TCP. No handshake. Fire and forget. That's why it's fast but also why DNS spoofing exists.

TCP: The Handshake

Got the IP. Now we connect.

Three packets before any data moves. 50ms to a server in another continent means 150ms just to establish connection.

This is why CDNs exist. Server closer = less RTT = faster handshake.

TLS: The Encrypted Handshake

HTTPS means another handshake. TLS 1.3 made this faster—one round trip instead of two. Saves 100ms+ on every new connection.

HTTP: Finally, the Request

Server processes. Could be instant for cached content. Could be 500ms if it's hitting a database.

Rendering: The Browser's Turn

HTML arrives. Browser starts parsing.

Parser hits a <script> tag? Stops. Downloads JS. Executes. Then continues. That's why script position matters. That's why async and defer exist.

Where Time Goes

Half is network. Quarter is server. Quarter is browser.

Optimize the wrong layer and you waste effort. SSR won't help if your images aren't compressed. CDN won't help if your server takes 2 seconds.

---

2 seconds. 14 steps. Dozens of ways to screw it up.

— blanho