What are the common TCP/IP Application Layer protocols and their port numbers?
Key ports: DNS 53, DHCP 67/68, SMTP 25, POP3 110, IMAP 143, FTP 20/21, TFTP 69, HTTP 80, HTTPS 443 — both hosts must run a compatible protocol to communicate.
A port number is how a host knows which application a packet is meant for: the well-known numbers below are fixed by convention so a client can reach the right service without being told. Learning them pays off twice — they show up constantly when reading firewall rules and packet captures, and the pairings hint at how each protocol works (UDP for the lightweight ones like DNS, DHCP, and TFTP; TCP for the ones that need reliability like the mail and web protocols).
Common Application Layer Protocols:
| Protocol | Port | Description |
|---|---|---|
| DNS | TCP/UDP 53 | Translates domain names to IP addresses |
| BOOTP/DHCP | UDP 67, 68 | Dynamic IP address assignment (BOOTP superseded by DHCP) |
| SMTP | TCP 25 | Sends email to mail servers |
| POP3 | TCP 110 | Retrieves email (downloads and deletes from server) |
| IMAP | TCP 143 | Retrieves email (keeps copy on server) |
| FTP | TCP 20, 21 | Reliable file transfer between client and server |
| TFTP | UDP 69 | Simple, connectionless file transfer |
| HTTP | TCP 80, 8080 | Web page transfer (not secure) |
| HTTPS | TCP/UDP 443 | Secure web page transfer with encryption |
Key insight: For successful communication, the application layer protocols implemented on source and destination hosts must be compatible.
Go deeper:
List of TCP and UDP port numbers (Wikipedia) — the full registry of well-known and registered ports.