An ephemeral port is a communications endpoint (port) of a transport layer protocol of the Internet protocol suite that is used for only a short period of time for the duration of a communication session. Such short-lived ports are allocated automatically within a predefined range of port numbers by the IP stack software of a computer operating system. The Transmission Control Protocol (TCP), the User Datagram Protocol (UDP), and the Stream Control Transmission Protocol (SCTP) typically use an ephemeral port for the client-end of a client–server communication. At the server end of the communication session, ephemeral ports may also be used for continuation of communications with a client that initially connected to one of the services listening with a well-known port. For example, the Trivial File Transfer Protocol (TFTP)^[1] and Remote Procedure Call (RPC) applications can behave in this manner.

The allocation of an ephemeral port is temporary and only valid for the duration of the communication session. After completion of the session, the port is destroyed and the port number becomes available for reuse, but many implementations simply increment the last used port number until the ephemeral port range is exhausted, when the numbers roll over. Ephemeral ports are also called dynamic ports, because they are used on a per request basis, and are only known by number once allocated.

If certain server software is used, that uses non-ephemeral custom port ranges for initiating some further connections, it needs to be ensured by configuration that this custom port range and the ephemeral port range do not overlap.^{[citation needed]}

• Registered port
• List of TCP and UDP port numbers