Portmap is server software running under Unix-like systems that converts RPC program numbers into DARPA protocol port numbers. Its design objective was to minimize the number of ports in use, but this never happened as it never had wide adoption. It must be running in order to make RPC calls. When an RPC server is started, it will tell portmap what port number it is listening to, and what RPC program numbers it is prepared to serve. When a client wishes to make an RPC call to a given program number, it will first contact portmap on the server machine to determine the port number where RPC packets should be sent. Portmap must be started before any RPC servers are invoked. The portmapper service first appeared in BSD 4.3 and has only been used extensively by NIS, NFS, and FAM so far.