An ephemeral port
is a TCP
or UDP port
number that is automatically allocated from a predefined range by the TCP/IP
stack software, typically to provide the port for the client
end of a client-server
communication. This allocation is temporary and is valid for the duration of the connection opened by the application using the protocols.
BSD used ports 1024 through 4999 as ephemeral ports, though it is often desirable to increase this allocation.
The IANA suggests 49152 to 65535 as "dynamic and/or private ports."