Do you ever find yourself needing to know whether an ethernet port will light up without actually hooking it up to a switch. Or, do you need to know what port on a switch a certain jack is plugged in to. Lucky for you, it’s really easy to make your own ethernet loopback cable.

To start, you’ll need a couple of tools: a wire cutter and a wire stripper. You’ll also need some electrical tape. And, last you’ll need an ethernet cable that you don’t mind cutting up.

Steps:

1. Cut about 12" off of the end of the cable
2. Strip off about 2" of the outer insulation
3. You should see 8 wires paired up (Orange, White-Orange, Green, White-Green, Blue, White-Blue, Brown, and White-Brown). Un-twist each pair.
4. Strip about an inch off of these four: White-Orange, White-Green, Orange, and Green
5. Wrap the exposed wires together matching White-Orange to White-Green and Orange to Green
6. Cover the exposed wire with electrical tape
7. You can ignore the other 4 wires
8. Plug the cable into either a network card or a switch. If it was done correctly the link light should light up.
9. Wrap the end of the cable with tape to "neaten" it up a little
10. Of course, you’ve still got the other end of the cable, so you might as well make another

I’ve seen this around the internet for anywhere from 5 bucks to 50 bucks. So, aside from the cost of a few feet of tape, a spare ethernet cable, and some time; you’ve saved yourself 50 bucks (or 100 if you made two :-) )