It is an additional 16-bit number which uniquely identifies the specific service on any given machine on the Internet. Port numbers are 16 bit wide, so each of the computers on the Internet contains a maximum number of 216 or 65,536 ports. The specific application is identified by its unique port number in the similar way that a particular television station has a unique channel number.

Port numbers are divided in three ranges which are following:

• Well-known ports are those from 0 to 1,023.
• Registered ports are those from 1,024 to 49,151.
• Dynamic and private ports are those from 49,152 to 65,535.

Well-known ports, those ranging from 0 to 1,023 are where on the Internet most common services are residing. These ports are controlled & assigned through the Internet Assigned Number Authority (IANA) and on most systems can be utilized only by system (root) processes or through programs executed by privileged users.