A socket address in the Internet namespace consists of a machine's Internet address plus a port number which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535.
Port numbers less than IPPORT_RESERVED are reserved for standard
servers, such as finger and telnet. There is a database
that keeps track of these, and you can use the getservbyname
function to map a service name onto a port number; see Services Database.
If you write a server that is not one of the standard ones defined in
the database, you must choose a port number for it. Use a number
greater than IPPORT_USERRESERVED; such numbers are reserved for
servers and won't ever be generated automatically by the system.
Avoiding conflicts with servers being run by other users is up to you.
When you use a socket without specifying its address, the system
generates a port number for it. This number is between
IPPORT_RESERVED and IPPORT_USERRESERVED.
On the Internet, it is actually legitimate to have two different
sockets with the same port number, as long as they never both try to
communicate with the same socket address (host address plus port
number). You shouldn't duplicate a port number except in special
circumstances where a higher-level protocol requires it. Normally,
the system won't let you do it; bind normally insists on
distinct port numbers. To reuse a port number, you must set the
socket option SO_REUSEADDR. See Socket-Level Options.