This will show you how to make this custom ethernet cable. This cable is intended to be used with a packet analyzer program like Wireshark, or Microsoft network monitor or messages analyzer.
I expect you to only use this for doing network troubleshooting, or learning about networking. This is read-only so you can't do as much with it, but you will be able to see some data. I accept no responsibility for your use or misuse of this cable.
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Step 1: What You Will Need
2 cat5 or better patch cables. Cat 5 will suffice for this and you may want to sacrifice that over cat5E or cat6
(I used this method found an old ~6FT cat5E cable, I had another cable with a bad end which became the read-only connection as it only needed one end.)
3 RJ45 connectors
About 9FT or so of cat5, 5E patch cable.
solder and solder iron or gun
(If useing option 2 RJ45 crimp tool)
Step 2: Prepare the Cables
Take the better cable and somewhere in the wire (I used the middle) remove about .5-1inch of the outer sheath by carefully using a razor or the diagonal cutter. Don't cut the inner wires, and avoid nicking these wires while removing the sheath.
Take the other cable and cut one of the ends off it. (I used a cable that had a bad end and cut the faulty end off)
Strip about .5-1inch of the outer sheath off the end of this cable. Seperate out the green pair of wires and strip a small amount of the insulation off the wire. Fold the others back or cut them off.
On the cable with both ends that you removed some outer sheath from separate out the orange pair so you can work on them. With a stripper or razor remove some insulation from both of the orange wires.
If you chose to do this with cable and RJ45 plugs:
Cut the desired amount of cable for the continuous cable (I'd say about 4-6FT) and about probly about 1/2 that for the read only cable. Crimp RJ45 plugs on both ends of the cable you want to have two connections on. Put one connector on the other cable. All the other steps are the same.
Step 3: Making Connections
Take the green wire from the cable with only one end and twist it around the orange wire of the cable with two ends.
Do similar with the green/white wire only twisting it around the orange/white sire instead.
Solder these two connections.
In the picture the gray cable is the read one, blue is the one with two ends. This picture was taken on my first try I realized I had to solder the green/white pair of the gray cable to the orange/white of the blue cable.
It still works if you match the colors, but you will only see traffic coming into the system instead of out.
Step 4: Finishing Touches
Take some electrical tape and wrap it around the solder joints you made this will prevent them from shorting out. Liquid electrical tape may be better if you have that.
Cover the area where you removed the outer sheath with a generous amount of electrical tape. About 1-2 inches from that where the read only lead comes out wrap that a couple times with tape this will keep it from pulling on the soldered joints and breaking.
Mark the read only lead with some sort of a label, put some colored tape around it so you know this is the read wire.
Step 5: Using the Cable.
You will out the read cable (the one that we only used the green pair from) to a system with Wireshark, etc installed.
The continuous part of the cable would go between whatever the device you want to monitor and whatever it was connected to.