You can specify a list of valid IP addresses by using Regedt32 to navigate to:
On the Edit menu, Add Value name AuthenticateIncomingCalls, as a type REG_DWORD. In the DWORD Editor, set the data to 1. The default is 0. Then, Add Value name PeerClientIPAddresses, as a type REG_MULTI_SZ. Type each valid IP address on a separate line in the Multi-String Editor.
Shutdown and restart your PPTP server.
NOTE: PeerClientIPAddresses is only used if AuthenticateIncomingCalls is set to a 1.
NOTE: If AuthenticateIncomingCalls is set to a 1 and PeerClientIPAddresses is missing or empty,
no clients will be able to connect.