Port numbers identify the each end of a network communication link and are established when the client opens a socket to connect to a server. Each port number must be unique within each host. The server processes for well-known application protocols also have well-known port numbers (so the client can connect to them easily). For example HTTP uses port 80, FTP uses port 21 for control and port 20 for data, telnet uses port 23, SMTP port 25, etc. Clients will typically be assigned port numbers in the range 1024–65,536 at random by the host operating system. For a full list of the well known port numbers see the list published by the Internet Assigned Numbers Authority (IANA) or the more user-friendly list maintained on Wikipedia. See also: socket.

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