The default for the RPC Endpoint Mapper is to bind to all interfaces.
Starting with SP5, you can use Regedt32 to navigate to:
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.