In computer networking a port is an application-specific or process-specific software construct serving as a communications endpoint in a computer’s host operating system. A port is associated with an IP address of the host, as well as the type of protocol used for communication. The protocols that primarily use the ports are the Transport Layer protocols, such as the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite. A port is identified for each address and protocol by a 16-bit number, commonly known as the port number. The port number completes the destination address for a communications session. Thus, different IP addresses or protocols may use the same port number for communication, e.g. on a given host or interface UDP and TCP may use the same port number, or on a host with two interfaces, both addresses are associated with a port having the same number. Be sure to grab a copy of The TCP/IP Guide: A Comprehensive, Illustrated Internet Protocols Reference.
