Ethernet Cross Over Cable

Two pairs crossed, two pairs uncrossed
10BASE-T or 100BASE-TX crossover



Gigabit T568A crossover
All four pairs crossed
10BASE-T, 100BASE-TX, 100BASE-T4 or 1000BASE-T crossover (shown as T568A)



Gigabit T568B crossover

All four pairs crossed
10BASE-T, 100BASE-TX, 100BASE-T4 or 1000BASE-T crossover (shown as T568B)
 



RJ45 Basic Loopback
 

1. Looking from the underside of the connector, CAT5 cable connects from bottom.
2. Connect a cable to pin 1 and 3.
3. Connect a cable to pin 2 and 6.
4. Crimp.

Full loopback (for Gigabit ethernet)

1. Looking from the underside of the connector, CAT5 cable connects from bottom.
2. Connect a cable to pin 1 and 3.
3. Connect a cable to pin 2 and 6.
4. (So far - same as the basic loopback).
5. Connect a cable to pin 4 and 7.
6. Connect a cable to pin 5 and 8.
7. Crimp.

source :
http://en.wikipedia.org/wiki/Ethernet_crossover_cable
http://sites.google.com/site/kalman/tech-break/ethernetrj-45loopback