Microsoft Information on configuring Windows NT as an IP router

Using Windows NT 3.51 or 4.0, either Workstation or Server, you can connect an entire LAN to the Internet over one modem. Microsoft publishes a comprehensive Technote, Article ID: Q121877, describing this process:
Using RAS for Routing of IP packets 
Note: one step is not clear, be sure that DisableOtherSrcPackets is 0
	 Registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\RasArp\
	           Parameters
	 Add Value name: DisableOtherSrcPackets
         Data Type: REG_DWORD
         Data: 0

There is also an older MS technote describing bugs in their other documentation. Try this query and read them for yourself:

Search MS NT Knowledge Base
Or, for convenience I have concatenated the documents here.

Reed Sturtevant <reed@sturtevant.com>
Last update: December 21, 1997
Sturtevant Technotes