How the web works

URLs, clients and servers, and the steps between typing an address and seeing a page.

The web is a conversation between a client (your browser, an app or a command like curl) and a server. They speak HTTP, the HyperText Transfer Protocol: the client sends a request, the server sends back a response.

Anatomy of a URL

https://www.example.com:443/guides/http?lang=en#headers
\___/   \_____________/ \_/ \___________/ \______/ \_____/
scheme       host      port     path        query   fragment

• scheme: the protocol, http or https.
• host: the server name, resolved to an IP address by DNS.
• port: optional, 80 for HTTP and 443 for HTTPS by default.
• path: which resource on the server.
• query: extra parameters after ?.
• fragment: a position inside the page after #. It is handled by the browser and never sent to the server.

What happens when you open a page

1. The browser asks DNS for the IP address of the host.
2. It opens a connection to that address on port 443.
3. For HTTPS, client and server agree on encryption (TLS) and the server proves its identity with a certificate.
4. The browser sends an HTTP request.
5. The server sends an HTTP response: a status code, headers and usually a body.
6. The browser reads the HTML and makes more requests for images, styles and scripts.

HTTP versions

HTTP/1.1 is text-based, one request at a time per connection. HTTP/2 multiplexes many requests over one connection. HTTP/3 runs over QUIC on UDP port 443. The semantics (methods, status codes, headers) are the same in all of them.

See it yourself

curl -v https://example.com    # -v shows the connection, TLS handshake, request and response headers