What Is A CrossOver Cable, and Why Do I Need One

In any conversation between two people or computers, you speak and the other listens. Or it speaks, and your computer listens. This means that your mouth has to connect to the ear on the other end. This is called cross-over.

If you look at any hub / switch / router 10 years ago, you would probably see the various ports labeled "X-1", "X-2", "X-3"... This meant those were cross-over ports. Your computer would speak (transmit) thru a pair of wires in the Ethernet cable. When the connection went into the router port at the other end, the cross-over function connected the transmit wire pair from your computer to the receive port at the other end, and the receive pair from your computer to the transmit pair at the other end.

If you had to connect a pair of routers directly to each other, you would have a cross-over port at one end connecting to a cross-over port at the other end. This would cause a cancellation of the cross-over function, so you would use a cross-over cable.

If you connected a pair of computers directly, you would similarly need a cross-over cable.

This meant that everybody with a computer network had to have cross-over cables handy.

To eliminate the need for using cross-over cables, router manufacturers developed Auto-MDIX. A router port with Auto-MDIX will listen to see if it is connected to another cross-over port, and switch itself to non-cross-over mode if necessary. Some computers, likewise, have Auto-MDIX. If you connect a pair of computers directly, and one (or both) have Auto-MDIX, you can use a straight-thru (aka patch) cable, and they will connect just fine.

Auto-MDIX is a significant development, in the networking world. Having said that, I don't believe that Auto-MDIX can be relied upon, as a complete solution. I will still advise you to have a cross-over cable, or connector, handy for diagnosing network problems.

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