A network port is a 16-bit number that identifies one side of a connection between two computers. Computers use port numbers to determine to which process or application a message should be delivered. As network addresses are like street address, port numbers are like suite or room numbers. Any program may use any port, though some port numbers have a standard use and some programs may be limited in which ports they can use for security reasons. Firewalls often block access to ports based on the network address and port of the source or destination computer, or the program using the port (if the firewall is running on the same computer).
