This post will be short (and sweet). Ethernet Loopback Adapters are little handy pieces of equipment that route the transmitting pins in an Ethernet jack back to the receiving pins in the same jack. This is good for testing link connectivity on an Ethernet card – if the adapter can establish a link with itself the hardware on the adapter is probably OK. In real life, this can save you hassle quickly testing ADSL modems, routers, switches, desktops and laptops without plugging the device into another jack to get the link light to come on.
This how-to will use an existing Ethernet cable which will be converted to a Loopback cable. There are lots of guides and video online about creating such an adapter using an Ethernet jack and wires, but this requires having an uncrimped Ethernet jack, some wires, and a crimper. In my case, a trip to the store to buy the components I already have at home on ready Ethernet cables seemed wasteful (most people will have a cable or two, or can buy a short cat 5 cable for under 2 dollars). I strongly recommend using a cable with a broken or missing jack – after all, we only need one Ethernet jack which is properly wired to a cat5 or higher cable.
1. Cut the cat5 cable a few ( 2 or 3 ) inches from the jack.
2. Strip about an inch from the shielding of the cat 5 cable, revealing 8 separately shielded color coded wires inside.
3. Strip about half an inch from the shielding on four wires: green-white, green, orange-white, and orange.
4. Twist the green-white and orange-white stripped ends together, connecting pin 1 to pin 3.
5. Twist the green and orange stripped ends together, connecting pin 2 to pin 6.
If you have some tape, you may want to cover the tips up. Otherwise, make sure the tips don’t touch each other. The end result will look like this:
twisted pairs on an Ethernet loopback adapter
You can test your new loopback adapter in any working Ethernet jack by plugging it in!
You can see the Port line is on for a wireless router with the loopback adapter plugged in:
Link light on with loopback adaper plugged in