Web protocols

Protocols are agreed rules for communication. Each one below works at the application layer and typically runs over TCP.

1. HTTP (HyperText Transfer Protocol)

• Purpose: transfers web pages and resources between browser and web server.
• Port: 80 (HTTPS = HTTP over TLS, port 443).
• Model: request/response and stateless.
• Methods: GET (retrieve), POST (submit), PUT (update), DELETE (remove), HEAD.
• Status codes: 200 OK, 301 redirect, 404 Not Found, 500 Server Error.

GET /index.html HTTP/1.1
Host: www.example.com

2. FTP (File Transfer Protocol)

• Purpose: uploading and downloading files between a client and a server (e.g. publishing a site to a host).
• Ports: 21 (control), 20 (data).
• Uses two connections (control and data), needs username/password authentication, and is unencrypted (use SFTP or FTPS for security).

3. SMTP (Simple Mail Transfer Protocol)

• Purpose: sending email from a client to a mail server and between mail servers (push protocol).
• Port: 25 (587 for submission with TLS).
• Handles routing and delivery of the outgoing message.

4. POP (Post Office Protocol, POP3)

• Purpose: retrieving email from the server to the client (pull protocol).
• Port: 110 (995 with SSL).
• Downloads messages to the local device, and normally deletes them from the server. Simple, but less suited to multiple devices than IMAP.

Comparison

Protocol	Purpose	Port	Direction
HTTP	Web pages/resources	80 / 443	Client ↔ web server
FTP	File transfer	20, 21	Client ↔ file server
SMTP	Send email	25 / 587	Client → server, server → server
POP3	Receive email	110 / 995	Mail server → client

Working together: the user browses with HTTP, uploads site files with FTP, sends mail through SMTP, and reads mail through POP3 (or IMAP).