What is the difference between a standard and cross over Ethernet cable?

Question

What is the difference between a standard and cross over Ethernet cable?

Answer 1

In early days of networking, Ethernet jacks were one of two types MDI or MDI-X.  MDI was used on the "host" end.  Hosts consisted of computers and routers (or any other endpoint device).  MDI-X jacks were used on switches and hubs for downstream connections.  Most switches/hubs of the time also had one jack for "uplink" which was an MDI jack.  To connect an MDI to and MDI-X jack, you used a straight through cable.  If you needed to connect an MDI to MDI or MDI-X to MDI-X then you had to use a crossover cable.  Basically, on an MDI jack pins 1/2 are the transmit pair and pins 3/6 are the receive pair.  In MDI-X, these pairs are swapped meaning 1/2 are receive and 3/6 are transmit.  A crossover cable is simply a cable where pins 1/2 on one end connector is connected to pins 3/6 on the other end connector and vice versa.  The analogy is a serial null-modem connector.  

Keep in mind that in todays networks, the distinction becomes moot in most cases since almost all networking devices as well as host interfaces are auto MDI/MDI-X which means that the two ports negotiate the right pairs to use.  If either or both ends do auto negotiation/detection it doesn't matter if you use a straight through or crossover cable.  In the case where neither end does auto negotiation where you may need a crossover cable if you are hooking two hosts directly together or two switches directly together.