How Port Numbers Work:

Port numbers are associated with network addresses. For example, in TCP/IP networking, both TCP and UDP utilize their own set of ports that work together with IP addresses.
Port numbers work like telephone extensions. Just as a business telephone switchboard can use a main phone number and assign each employee an extension number (like x100, x101, etc.), so a computer has a main address and a set of port numbers to handle incoming and outgoing connections.
In both TCP and UDP, port numbers start at 0 and go up to 65535. Numbers in the lower ranges are dedicated to common Internet protocols (like 21 for FTP, 80 for HTTP, etc.).