An ephemeral port, also called a transient port or temporary port, is a temporary, short-lived port number assigned by an operating system to a client application for the duration of a communication session with a server. These ports are used as communication endpoints in transport layer protocols such as TCP, UDP, or SCTP and are essential for enabling multiple simultaneous client-server connections without port conflicts.
When a client wants to communicate with a server (e.g., accessing a website), the server listens on a known port (like 80 for HTTP or 443 for HTTPS). The client’s operating system assigns an ephemeral port as the source port for this connection. This port acts as a temporary return address for the server’s responses. Once the communication session ends, the ephemeral port is released and returned to the pool for reuse in future connections.