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A port mapper is the protocol that maps the number or version of an Open Network Computing Remote Procedure Call (ONC RPC) program to a port used for networking by that version of the program. Upon initiation, the ONC RPC server requests the port mapper to assign a port number for each program using the transport protocol for file transfer or communication purposes. Therefore programs use a port mapper to determine which port is assigned to them before using it.
The port mapper assigns a unique TCP/UDP protocol port number to an RPC program. Upon starting, the Network File System uses a port map to listen to and send data to specific ports. Applications or processes that use the TCP/UDP protocol also use a port mapper, which assigns them a unique port number that they can use to make a connection. This is especially useful in ONC RPC servers where the port mapper determines the ports to be used upon initiation. The port mapper must always start before any other RPC server is started. Several developers have developed port mappers that perform port assignment for programs requiring it. Since a port mapper works on ports, its main work is in the transport layer.