In order for server to receive connection of client, it must have something called listening port. Listening port is made of host address and port. Host address is an internet IP address in the format of 111.222.111.222 or mycomputer,mydomain.com. Port is value between 1,000 to 65,500.

As lomg as more than two programs use the same listening port, you can appoint a listening port in any way you like.

An entrance is needed to enter. Listening port works as an entrance for clients to enter server.”