$nmap -p- protocolports.org
Every TCP & UDP port, explained.
Look up any port to see what it is, what software uses it, and how to open or forward it — plus the security basics.
Popular ports
Echo Protocol
Legacy diagnostic service that echoes back any data it receives, used for connectivity testing.
CharGEN (Character Generator Protocol)
Character Generator — a legacy debugging service that emits a stream of characters in response to any input.
FTP (File Transfer Protocol) data channel
FTP active-mode data channel that carries file contents and directory listings.
FTP (File Transfer Protocol) control channel
FTP command and authentication channel for browsing and transferring files.
SSH (Secure Shell)
Secure Shell — encrypted remote login, command execution, and tunneling.
Telnet
Unencrypted remote terminal protocol for interactive login and device management.
SMTP (Simple Mail Transfer Protocol)
Server-to-server email transfer protocol used to relay and deliver mail between MTAs.
TACACS+ (Terminal Access Controller Access-Control System Plus)
TACACS+ — Cisco's AAA protocol that authenticates and authorizes administrative access to network devices.