Workshop

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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.)