The default for the RPC Endpoint Mapper is to bind to all interfaces.

Starting with SP5, you can use Regedt32 to navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RPCSS

On the Edit menu, Add Value name ListenOnInternet, a type REG_SZ entry, and set the string as follows:

Y - RPCSS listens on all ports. This is the default.

N - RPCSS listens based on the current policy.