# Port Number

A port number is logical channel or channel endpoint in a communications system. The Transmission Control Protocol and User Datagram Protocol transport layer protocols used on Ethernet use port numbers to distinguish between (demultiplex) different logical channels on the same network interface on the same computer.

Each application program has a unique port number associated with it, defined in /etc/services or the Network Information Service "services" database. Some protocols, for example, Telnet and HTTP (which is actually a special form of Telnet) have default ports specified as above but can use other ports as well.

The FTP client must know which port to connect to. The port number is already assigned and listed on the IIS FTP Service Overview Page.

- Note: To access the FTP server on the domain, you must use a port number other than port 21. If you do not specify a port number, the FTP Server client defaults to port 21.