How to create a RJ-45 loopback adapter
When testing a router that has not been cabled to the network, it is possible to build a loopback adapter out of a single RJ-45 connector and some wire. I recommend that every networker carry one or two of these beasties with you at all times. They come in handy for servers as well.
With the pins facing towards you and starting from the left, the pins on an RJ-45 jack are numbered from 1 to 8. You will need two very short pieces of wire.
1. Wire pin 1 to pin 3.
1. Wire pin 2 to pin 6.
Do not wire any of the other pins.
You are done! Ensure that you have a good crimp on the wires.