IP Routing with NT

Last month, I talked about the basics of Internet Protocol (IP) addressing. The reason that anyone cares about IP is because it makes building large networks relatively simple. The strength of IP lies in its routing ability. But in order to use IP for this purpose, you need a piece of hardware called an IP router.

There are several kinds of IP routers: One connects your network to the Internet over a WAN connection. Another connects LANs to LANs directly with no WAN connections.

The first kind of router you'll need to install is the network-to-WAN router, but as your network grows, you'll also need the LAN-to-LAN type. The problem with network-to-network routers is that they cost about $3000+. But for simple routing, you don't need to spend a dime: You can use Windows NT Server. This month, I'll show you how.

LAN-to-WAN Routers
My firm uses about 50 IP addresses for its computers (most of which need one IP address each), our Integrated Services Digital Network (ISDN) bridges (again, one IP address each), and our routers. A few years ago, we put our network on the Internet. To do that, we needed a LAN-to-WAN IP router and an Internet Service Provider (ISP).

A router is basically a box that sets between two networks. It allows each network to communicate with the other by relaying messages between them. For example, on my network, I'm allowed to use the range of addresses from 199.34.57.0 through 199.34.57.255--I have a C-class network.

You may recall from last month that I cannot use two of those addresses: 199.34.57.0 and 199.34.57.255. The ".0" is the network number, a generic way of saying "all addresses from 199.34.57.1 through 199.34.57.254." The ".255" is the broadcast address, a single address used to send a message to all the computers on my network. (Last month I also noted that I couldn't use the 199.34.57.1 and 199.34.57.2 addresses. That's not a carved-in-stone Internet rule, but rather an artifact of how I run my network.)

My network resembles the network shown in figure 1. It has a LAN-to-WAN IP router with a set of instructions that tells it how to route the packets it receives. The routing table has an absurdly simple mission: If there's a message for 199.34.57.0--in other words, if there's a message for any address on my C-class network--send it to the Ethernet. If there's a message for any other address, send it via the serial port to the modem, which then puts the message on the Internet.

The router in figure 1 is not the one I recommend you use with Windows NT Server. It's not usually a good idea to use Windows NT as a LAN-to-WAN router, although it can be done.

For my network, I use a Compatible Systems Microrouter 900i. It's one of the best purchases I ever made for my network. It cost about $850, which is quite reasonable, but that's not the only reason it's a good deal. Although it's possible to use an NT machine as an Internet-to-Ethernet-via-WAN-connection router, my tests show that the Compatible box routes packets more than twice as fast as an NT machine does.

Building an NT LAN-to-LAN Router
If my network becomes larger and I get another C-class network, for example, 223.200.200.0, or if I decide to divide my network into smaller subnets, I will have multiple network segments--it's the same story whether you use Ethernet or Token Ring--and I will need a router (see figure 2).

The LAN-to-WAN router now has a more difficult job. For a machine on 199.34.57.2 to send a packet to any machine on 223.200.200.0, it must send the packet to the router at 199.34.57.2. That's where it sends all packets that need routing. Now that router must be smart enough to know not to send the packet over the WAN but to send it to the router for 223.200.200.0. You also need a LAN-to-LAN router between 199.34.57.0 and 223.200.200.0.

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