Port Number

When a client process wants to contact a server process, it must have a way of identifying the server that it wants. It is not enough that the client only knows the 32-bit IP Address of the host on which the server resides because there might have many servers running on the same host. Port Number is used to indentify each server. Some ports (below 1024) are well-defined for some specific servers. Such as, 21 for FTP, 23 for Telnet, 80 for HTTP, etc....