Routing TCP/IP Packets On a Multihomed RAS Computer

A TCP/IP workstation can be configured to use only one default gateway, which may present a challenge to users that are connected to a local TCP/IP network through a network interface card (NIC) and to a remote network using Windows NT Remote Access Services (RAS).

NOTE: A computer connected to more than one LAN segment in this way is known as a multihomed system.

If you are running Windows NT and you computer is connected to both a remote and local TCP/IP network and you attempt to access an IP address, the destination host is located using the following process:

1. If you have enabled the option to "Use default gateway on remote network" in the TCP/IP options of the RAS PhoneBook entry used to dial the remote network:
   1. If the destination IP address is on the same subnet as your computer's NIC, the packets are sent via the NIC.
   2. If the destination IP address is not on the same subnet as your computer's NIC, the packets are sent via your RAS connection to the default gateway assigned by the RAS server.
2. If you have not enabled the option to "Use default gateway on remote network" in the TCP/IP options of the RAS PhoneBook entry used to dial the remote network:
   1. If the destination IP address is on the same subnet as your computer's assigned RAS IP address, the packets are sent via the RAS connection.
   2. If the destination IP address is not on the same subnet as your computer's assigned RAS IP address, the packets are sent via your computer's NIC.

This may present a problem to some users. For example, if your LAN is divided into two Class C subnets, 1.1.1.x and 1.1.2.x, and you configure your computer on the 1.1.1.x subnet to use RAS with the "Use default gateway on remote network" option enabled, all packets you send to the 1.1.2.x subnet will be sent via the RAS connection and not your computer's NIC.

To workaround this problem, you can use the Windows NT ROUTE command to add a static route to your TCP/IP route table that will instruct Windows NT to send packets intended for the 1.1.2.x subnet to a router on the 1.1.1.x subnet. For more information on using the Windows NT ROUTE command, refer to the Windows NT Command Reference section of the Windows NT Help file (WINHLP32.EXE).

"Microsoft Windows NT Resource Kit, Vol. 2, Windows NT Networking Guide," Pages 414 and 415, Microsoft Press, 1995.