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Hour 24
Reverse-Engineering Somebody Else’s Network

Congratulations. You now know enough to reverse-engineer somebody else’s undocumented network. I’m so proud!

You’ve probably noticed that documentation has been a major theme of this book. And why not? If you’ve turned ahead to this hour out of desperation, you may well be the victim of somebody else’s undocumented network, and you have a good idea of how frustrating it can be. Fret not. This is where we’re going to comb the tangles out of that undocumented network and make it into a reliable and manageable beast. It’s going to be humming along by the time you get through.

Grab a sharp pencil and take a deep breath, you’re about to chart uncharted territory. We’ll start off with physical cable tracing and then move on to TCP/IP and IPX/SPX networks. (Why leave out NetBEUI? Remember, NetBEUI is a very simple, nonroutable protocol—it can’t be too complex, and a physical cabling diagram is probably enough.) We’re going to assume that you’ve come into possession of a working network; it’s too tough to reverse-engineer a network that’s broken. So, let’s hurry and document it before it goes down. The network is up, and today’s the day.

Cable Tracing

An unlabeled cable is a troubleshooter’s nightmare. After all, it’s in the wall, making it hard to know where the heck it’s going. Fortunately, most sites only have a certain number of electrical closets, and that narrows the number of locations that you’ll need to search.

One tool is an absolute must if you’re trying to trace (and label) unlabeled cables. For under $100, you can get an inductive tone generator and an inductive tone tracer. (Jensen Tools, at www.jensentools.com, is one supplier, but there are others.) The theory behind an inductive tone generator is that it generates such a strong signal on the wire that the tracer can “hear” it—even without touching the wire. Very cool. This allows you to quickly and easily trace a wire from one end to another.

The operation of the generator/tracer pair is pretty simple. Follow these steps:

1.  Identify the wall jack or cable end that you want to start with. (I start with “far end” stuff, for reasons you’ll see in a minute.)
2.  Hook the tone generator to the cable (make sure to follow the wiring directions that come with it). Then turn it on.
3.  Take the tone tracer and do a quick test to make sure everything is working: Follow the wire a couple of feet down, and make sure you can hear a tone—remember that you shouldn’t have to touch the cable to hear the tone. Notice how the tone gets stronger as you move toward the cable.
4.  Take the tone tracer to a site where most cables seem to “terminate” and wave the tracer near all the cables. If you hear a tone, you’re in the right area. Play “hot and cold” (as the tone gets louder, you’re closer to the right cable; as it gets softer, you’re getting away from the cable).
5.  Repeat this at different closets, hubs, and so on until you find the cable.
6.  Label it.

You now see why you put the generator at the workstation end. You want to limit the number of places you have to visit with the tracer.

Of course, if you have a very small site with only one or two hubs, you can always have a friend turn a computer off while you’re looking at the hub. The hub port light that goes out belongs to the PC that just got turned off. Now you can label the wire appropriately. Still, inductive toners are a lot of fun to use.

TCP/IP Discovery

Your first task in discovering how a TCP/IP network is laid out is to identify the “glue” of the network—the routers! The easiest way to discover the routers on your network is to go to a functional workstation and check the TCP/IP configuration. Go to several in different locations, particularly if you’re not sure whether you have multiple segments.


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