Crossover cables -- 568A (or 258B) is a wiring configuration that is the exact opposite of the standard 568B (258A), which is why it is called a crossover cable. Put another way, the wires that are designated for transmitting and receiving are crossed over. One example of when you would want to use a crossover cable is when you want two computers to talk to each other without the use of a hardwired network.

The crossover adapter will allow for two computers to communicate by crossing a signal sent from pin 2 (designated for sending) over to pin 6 (designated for receiving) on the opposite end of the cable which is connected to the second computer.