This value is normally port number 80, but ports 81, 8080 or 8081 are also commonly used. It can be any port number you like although there are many reserved ports for other services on the host. On a Unix system, there is an /etc/services file that will enumerate the ports that are likely to be reserved for other non web-server purposes.

You may also have difficulty in using ports below number 1024 unless you have access to the system administrator account.