They allow specific communications between computers, applications and programs.
A port is how a program, computer, application communicates with each other using TCP or UDP.
The ports are broken down into three categories:
- Well Known Ports: Range from 0 through 1023 and are assigned by the IANA (Internet Assigned Number Authority). These are the most commonly used ports.
- Registered Ports: Range from 1024 through 49151 and are listed by the IANA. These ports can be used through normal user processes or executable programs on most computers.
- Dynamic and/or Private Ports: Range from 49152 through 65535.
For a complete list of all the TCP/UDP ports please visit Iana Port numbers or Network Ice Port Knowledgebase.