An ephemeral port, also called a dynamic port, is an automatically allocated temporary port, usually on the client side, opened when a client application initiates a Transmission Control Protocol/Internet Port (TCP/IP) or User Datagram Protocol (UDP) communication.
The temporary, or ephemeral, port is assigned in a semi-random manner from a range of lesser used ports reserved specifically for that purpose. Different operating systems assign different port ranges for ephemeral ports. When the client application terminates or times out, the ephemeral port is available for reuse. To ensure client-server communications are not interrupted, ephemeral post numbers are reassigned based on a set of rules created in the OS.
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