Port (computer networking)

In computer networking, a port is an endpoint of communication in an operating system. While the term is also used for hardware devices, in software it is a logical construct that identifies a specific process or a type of service.

A port is always associated with an IP address of a host and the protocol type of the communication, and thus completes the destination or origination address of a communications session. A port is identified for each address and protocol by a 16-bit number, commonly known as the port number.

Specific port numbers are often used to identify specific services. Of the thousands of enumerated ports, 1024 well-known port numbers are reserved by convention to identify specific service types on a host. The protocols that primarily use ports are the transport layer protocols, such as the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet protocol suite.

Ports are unnecessary on direct point-to-point links when the computers at each end can only run one program at a time. Ports became necessary after computers became capable of executing more than one program at a time and were connected to modern packet-switched networks. In the client–server model of application architecture, the ports that network clients connect to for service initiation provide a multiplexing service. After initial communication binds to the well-known port number, this port is freed by switching each instance of service requests to a dedicated, connection-specific port number, so that additional clients can be serviced.