from
The Free On-line Dictionary of Computing (8 July 2008)
portmapper
<networking> A {server} that converts {TCP/IP} {protocol}
{port} numbers into {RPC} program numbers. It must be running
in order to make RPC calls.
When an RPC server starts, it tells portmap the port number it
is listening on and what RPC program numbers it serves.
Before a client can call a given RPC program number, it must
contacts portmap on the server machine to determine the port
number to which RPC packets should be sent.
(1996-12-09)