Remote Procedure Call Port Mapper
Since MSRPC services are running on Ephemeral Ports it is the port mappers job to give clients information on available services and their port numbers.
The RPC port mapper is a service from Microsoft that is constantly listening on Port 135 and to tell tell connecting devices which MSRPC services are available and on which ports they are accessible.
Enumeration
If you want to see the information that the port mapper supplies use this Nmap command option.
nmap <IP> --script=msrpc-enum
Warning
You will see many entries when enumerating this way with Nmap because you will see services that not only are available remotely but also locally on the machine. You will notice that some of the services don’t have port numbers.