(or port address) To ensure that each server application responds only to requests and communications from appropriate clients, each server is assigned a port address. If IP addresses are like street addresses, then ports can be thought of as apartment or suite numbers. Common IP Port Numbers 20 21 23 25 53 70 80 107 109 110 119 137 143 194 220 389 443 514 540 FTP (File Transfer Protocol) FTP (File Transfer Protocol) Telnet SMTP (Simple Mail Transfer Protocol) DNS (Domain Name Server) Gopher HTTP (Hypertext Transfer Protocol) Remote Telnet Service POP2 (Post Office Protocol version 2) POP3 (Post Office Protocol version 3) NNTP (Network News Transfer Protocol) WINS (Windows Internet Naming Service) IMAP4 (Interactive Mail Access Protocol 4) IRC (Internet Relay Chat) IMAP3 (Interactive Mail Access Protocol 3) LDAP (Lightweight Directory Access Protocol) SYSLOG HTTPS (HTTP running over secure sockets) UUCP (UNIX to UNIX Copy)