Windows NT 4.0 would lease enough IP addresses for all RAS devices configured on the the RAS server plus one for the RAS server interface. If you had 20 modems configured on the Windows NT 4.0 RAS server, it would lease 21 IP addresses when the RAS service starts.
When DHCP can not be contacted by a Windows NT 4.0 RAS server, it does not provide IP addresses to the client, which prevents the client from connecting using TCP/IP.
In Windows 2000, the RAS server leases IP addresses in blocks of 10 and stores then in the registry. When additional additional IP addresses are required, it leases 10 more. You can alter the number of IP addresses leased per block by using Regedt32 to navigate to:
Edit or Add Value name InitialAddressPoolSize as a REG_DWORD data type. Set the data value to the number you want leased per block.
When a Windows 2000 RAS server can not contact a DHCP server, it uses Automatic Private IP Addressing (APIPA) to assign IP addresses to RAS clients.