Building a Multihomed Router

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Building a Multihomed Router

Windows NT enables an administrator to convert a machine into either a static or dynamic IP router. Static routers work well for extending a small network segment; dynamic routers using RIP work well on small to medium networks. A multihomed computer would probably not work well on large networks, however, based on RIP’s limitations and the significant overhead associated with maintaining large route tables. Other considerations aside, however, building an NT router is fairly simple and easy to do.

Before continuing, let’s define a or multihomed router. A multi-homed router is simply a computer with more than one network card that has been configured to route packets from one network segment to another. The defining characteristic between a hardware router and a multihomed router is that on a multihomed computer, the operating system is the one that performs the routing. A hardware router is a device that is specifically manufactured and designed for routing only. You could think about it in more simple terms. For instance, you can run any Windows application, including Freecell, on a multihomed router; you cannot on a hardware router.

The first step toward building an NT router is to install two or more network cards in the machine. Anyone who has ever tried to do so will tell you this can often sound much easier than it is. Each network card has to have its own IRQ and I/O address to use on the machine. These must be independent of other hardware cards you may be using in your machine, including video cards, sound cards, modems, hard disk controller cards, and so on.

Basically, the machine needs to be stripped of any bells and whistles and other functions so that enough resources are available. Any resource conflicts result in significant headaches as your network cards don’t appear and protocol drivers fail to load. The typical machine built for NT seminars and classes utilizes an NT router with three network cards and little else. After the machine successfully identifies the network cards, be careful of installing any additional third-party utilities. Sometimes they decide to steal the I/O addresses your network cards are using. The bottom line is that once this machine is built, try to leave it alone. Getting your machine stable will be the toughest part. Afterward, everything else is easy.

Be careful when installing third-party utilities after the router is configured. Sometimes they steal I/O addresses that may conflict with your network cards and cause routing problems.

After installing the network cards, make sure to assign separate IP addresses to each card, as follows:

1.	In the network section of Control Panel under the protocol tab. Select TCP/IP and choose properties. Notice that where the network card is identified, the drop-down box reveals all the network cards you have installed, enabling you to choose a different IP address scheme for each network card.
2.	After you give each network card its own IP address, indicating which network it is on, the machine can respond to packets coming from the networks to which it is attached. However, the machine is still not a router.
3.	To turn the machine into a router, go back to TCP/IP properties and choose the routing Tab. Select the Enable IP Forwarding check box. After you select this box and have chosen OK to exit this configuration and the network configuration, you are asked to reboot your machine.
4.	Reboot the machine. After the machine is rebooted, it is officially a router that can pass packets from one network to another.

The administrator then needs to decide whether the router will be static or dynamic. After IP forwarding is enabled, the router is a static router. If this is what is desired, no more configuration is necessary. If the administrator want to make this a dynamic router, then the RIP protocol needs to be installed.

This can be installed in the Services Tab through the network icon. After RIP is installed, this router listens for other RIP broadcasts, and broadcasts its own route table entries.

Although Windows NT supports the capability to create a static or dynamic router, the most important consideration for an administrator is probably whether he or she should spend the money to upgrade a machine for occasional routing of packets or spend the money for a hardware router. If the administrator plans to spend over $1,000 for a machine to route packets on a network, he may be better off spending it on hardware optimized for that purpose. Think of Windows NT routing versus hardware routing in much the same way as you would think about Windows NT RAID versus hardware RAID. Hardware implementation is usually a little more expensive, but is optimized for that specific task, whereas Windows NT implementations work well and are cheaper, but are not designed for constant pounding by a large network.

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