Introduction to HTTP

Every time you click a link, a small conversation takes place between two machines. One asks a question; the other answers it. The language they speak is HTTP, and it is the most widely used application protocol on Earth. Billions of requests flow through it every hour—​web pages, images, API calls, video streams—​all carried by the same simple exchange of text messages that Tim Berners-Lee sketched on a notepad in 1990.

You are about to learn that language from the ground up.

We start with the basics: what HTTP actually is, and how clients and servers find each other through URLs. From there you will look inside the messages themselves—​their structure, the methods that give them purpose, and the status codes that report what happened. You will see how headers quietly orchestrate everything from content types to caching policies, and how content negotiation lets a single resource serve different representations to different clients.

Then the picture gets more interesting. You will learn how connections are opened, reused, and closed—​and why getting this right matters more than most people realize. Caching will show you how the Web avoids doing the same work twice. Authentication will reveal how identity and trust are woven into the protocol without breaking its stateless design.

Finally, you will follow HTTP’s evolution into its modern forms: the binary multiplexing of HTTP/2, and the QUIC-based transport of HTTP/3 that eliminates decades-old performance bottlenecks at the transport layer.

None of this requires prior networking experience. Each section builds on the last, and by the end you will read raw HTTP traffic the way a mechanic reads an engine—​seeing not just what is happening, but why.