How do I configure NT to be a gateway to an ISP?

A. Firstly the hardware required would be a network and a modem. The network card would be so the other clients in the network can communicate with the "to be" gateway, and the modem to connect to the gateway. Dial-up networking is not covered here, and you should first be confident with dial-up networking before attempting this.

  1. Start the registry editor (regedit.exe) and add a value of type DWORD called DisableOtherSrcPackets in the HKey_Local_Machine\System\CurrentControlSet\Services\RasArp\Parameters area, and set to a value of 0. This is so packets that are sent through the NT gateway, the original IP address stored in each packet is retained, i.e. of machine a is sending a packet through b, then the packet retains the IP address of a, rather then be automatically changed to b. Also change HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\IPEnableRouter to a value of 1.
  2. On the gateway machine ensure TCP/IP is installed with a static IP address, and a correct subnet address (usually for a class a, for class b, and for class c). Make sure the default gateway address is blank.
  3. Install Dial Up networking and configure for NT to dial out only. You will have to reboot
  4. Add a phonebook entry for your ISP as you would as normal, however uncheck the "Use default gateway".
  5. Enable the PC to be able to forward IP packets, by starting control panel, double click Network and choose the protocols tab. Select TCP/IP and then routing. Check the Enable IP Forwarding. You will need to reboot
  6. If when you connect to your ISP you are given an IP address, you will need to connect to your ISP, and then find out which IP address you are given. To get the address type
    Look for a Wan adapter and write down the IP address. If you know your IP address before you connect you can forget this step.
  7. Add a route for the IP address used when connecting to the ISP (the one identified in step 6)
    route add mask <ip address> metric 2
  8. Configure all clients gateway as the network card IP address of the NT gateway.

This would enable the machines to send out IP packets to the internet, however the packets would have no way of finding there way back, as the ISP would not know to route them through the gateway, so you ISP will have to either a) have host entries for each of the machines or b) point to the gateway as another DNS.

Other things to check are as follows:

  • Make sure your ISP routes packets to you otherwise you will be able to send packets out but the replies will never be get to you
  • Make sure your local IP networks (each machine can ping each other) and that all PC's have a valid internet address. If you do not have internet addresses for each PC that have been assigned from InterNic then you will need something like Proxy Server instead.

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