TCP And UDP Ports

In order to allow targeting a specific application on a given host, the TCP and UDP protocols add port numbers. These are numbers ranging between 0-65535.

Ports 0-1023 are privileged and may be opened for listening only by privileged applications (running as "root" under Unix). Ports above 1023 may be opened for listening by any user on the host.

Thus, the address of a given application is a combination of the host's IP address, and an assigned port number. Example: 204.141.75.77:23 .

In order for a client to find a given server, it must know in advance which port the server listens on. Common services use "well known ports", such as 23 for telnet, 21 for ftp, 80 for http, etc.