The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) needed only one port for full-duplex, bidirectional traffic. The Stream Control Transmission Protocol (SCTP) and the Datagram Congestion Control Protocol (DCCP) also use port numbers. They usually use port numbers that match the services of the corresponding TCP or UDP implementation, if they exist.
The Internet Assigned Numbers Authority (IANA) is responsible for maintaining the official assignments of port numbers for specific uses. However, many unofficial uses of both well-known and registered port numbers occur in practice. Similarly many of the official assignments refer to protocols that were never or are no longer in common use. This article lists port numbers and their associated protocols that have experienced significant uptake.