This is a list of Internet socket port numbers used by protocols of the transport layer of the Internet Protocol Suite for the establishment of host-to-host connectivity.

Originally, port numbers were used by the Network Control Program (NCP) in the ARPANET for which two ports were required for half-duplex transmission. Later, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) needed only one port for full-duplex, bidirectional traffic. The even-numbered ports were not used, and this resulted in some even numbers in the well-known port number range being unassigned. The Stream Control Transmission Protocol (SCTP) and the Datagram Congestion Control Protocol (D***) also use port numbers. They usually use port numbers that match the services of the corresponding TCP or UDP implementation, if they exist.