Crossover cables are used to connect two Ethernet cables with each other, they were specially created for a host to host situations.
Standard cables have an identical sequence of colored wires on each end. Crossover cables have the first and third wires (counting from left to right) crossed, and the second and sixth wires crossed
A good Ethernet crossover cable will be specially marked to distinguish it from straight through ones. Many are red in color and also feature the name “crossover” stamped on its packaging and wire casing. Crossover cables were commonly used by Information Technology (IT) professionals in the 1990s and 2000s as the popular forms of Ethernet at that time did not support direct cable connections between hosts. Both the original and Fast Ethernet standards were designed to use specific wires for both the transmit and receive signals.
These standards required the two endpoints to communicate through an intermediate device to avoid conflicts from trying to use the same wires for both transmit and receive. A feature of Ethernet called MDI-X provides the necessary auto-detection support to prevent these signal conflicts.
This feature allows the Ethernet interface to automatically determine which signaling convention the device on the other end of the cable expects and negotiates use of the transmit and receive wires accordingly. Note that only one end of a connection needs to support auto-detection for this feature to work.
Most home broadband routers (even older models) incorporated MDI-X support on their Ethernet interfaces. Gigabit Ethernet also adopted MDI-X as a standard. In summary, crossover cables are only needed when connecting two Ethernet client devices where neither is configured for Gigabit Ethernet. Modern Ethernet devices now automatically detect the use of crossover cables and work with them seamlessly. Crossover cables should only be used for direct network connections.
For the reason described above, attempting to connect a computer to an old router or network switch with a crossover cable instead of a normal cable can prevent the link from functioning. These cables can be specially purchased through various electronics outlets. Hobbyists and some IT professionals may prefer to make their own crossover cables instead.