Occasionally I have the need for a network card to operate as though it has a live network connection, without it actually being connected to a network. Typically for testing purposes, and occasionally for specific software configurations. To do this I need a quick and easy way to make a loopback connector. Something small enough to keep in my PC toolkit, yet large enough that it won't be lost. Mine end up about the size of a ballpoint pen. The following is a quick "how to" for making an Ethernet loopback connector.

To build this particular loopback connector you will need one (1) spare Category 5e (cat5) cable.
1. Cut 4 or 5 inches of the end off of a network cable, keeping the connector intact.
2. Cut away two inches of the main sheath covering the eight wires. (Be careful not to cut the wires inside.)
3. Cut the sheath on Orange-White and Green-White and twist them together. Tape this pair. (Soldering is recommended)
4. Cut the sheath on Green and Orange and twist them together. Tape this pair as well. (Soldering is recommended)
5. Cut the sheath on Blue and Brown-White and twist them together. Tape this pair as well. (Soldering is recommended)
6. Cut the sheath on Blue-White and Brown and twist them together. Tape this pair as well. (Soldering is recommended)
7. Tape the end down to make it look presentable.
8. Plug your new RJ-45 loopback into your Network Card.

Here are some extra references:

http://logout.sh/computers/net/gigabit/

http://en.wikipedia.org/wiki/Ethernet_crossover_cable

http://docs.sun.com/source/817-7665/AppendixA.html