Tech Note: Configuring Windows as a router

Configuring Windows NT 4.0 as a router

I am using a PC running Windows NT 4.0 Workstation, Service Pack 3, as a router to connect several machines to the Internet over a single modem. To do this, your Internet access provider must give you a block of IP addresses that you can assign to the machines on your network. Microsoft publishes a comprehensive tech note describing the requirements and configuration: Article ID: Q121877 Using RAS for Routing of IP packets 
Note: that documentation has one step that's not clear, be sure the 
registry key DisableOtherSrcPackets is set to 0
	 Registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\RasArp\
	           Parameters
	 Add Value name: DisableOtherSrcPackets
         Data Type: REG_DWORD
         Data: 0

You should check out a diagram of my network configuration, which consists of three PCs.

Configuring Windows NT 3.51 as a router

Using Windows NT, you can connect an entire LAN to the Internet over the same modem. I got things running pretty easily once I cobbled together the following instructions. These describe how I configured my system when I was running NT 3.51, service pack 5. Before you begin, you should be relatively comfortable with Windows NT system management and TCP/IP concepts.

1) get your access provider to route a block of addresses to you
2) configure your RAS connection
3) configure TCP/IP settings for your network card. Leave the default gateway blank
4) edit registry settings on your router machine to add values as follows:

HKEY_LOCAL_MACHINE
    \System\CurrentControlSet\Services\RasArp\Parameters\DisableOtherSrcPackets
    Data type REG_DWORD, value = 0
HKEY_LOCAL_MACHINE
    \System\CurrentControlSet\Services\Tcpip\Parameters\IPEnableRouter
    Data type REG_DWORD, value = 1
5) On other machines on your LAN, set gateway to the IP of the machine used as the router.
6) NOTE: if you have a recent NT Service Pack installed, you must have the IP addresses of your LAN on a different subnet than your incoming RAS connection. For instance, let's say your service provider routes packets to address xxx.xxx.xxx.1 (your incoming RAS connection on the router PC). Configure the ethernet card on that PC to be xxx.xxx.xxx.129 and use a Subnet Mask of 255.255.255.128. Give the other PCs on your LAN IP addresses above 129 and the same subnet mask.

You are all set. When the router machine is online, dialed into your access provider, it will route IP packets to and from any other machine on your network. In fact, you can configure additional modems on the router as RAS dial-in ports...you are then an Internet access provider in your own right!

For more information, check out Mike Jacobs instructions for NT 4.0 and what Microsoft says about RAS Routing or another description by Somar Software.

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