Ethernet network interface cards in end devices like PCs, servers and IP cameras are referred to as MDI (medium dependent interfaces). When the MDI communicates with a network device like a hub, switch or router these MDI devices transmit on pins 1,2, 4 and 5 and receive on pins 3, 6, 7 and 8. Hubs, switches and routers (infrastructure nodes) connect using MDI-X and transmit on pins 3, 6 7 and 8 and receive on pins 1,2, 4 and 5. To enable a connection between a MDI and MDI-X device a straight through cable is required.

Nodes can have two types of ports, MDI (Uplink port) or MDI-X (Regular Port, 'X' for internal crossover). Hubs and switches have regular ports. Routers, servers and end hosts (e.g. personal computer) have uplink ports. If you want to connect two devices port with the same type of port for 10 or 100 Mbit/s connections then a crossover cable is required.

Many of today’s network interface cards automatically change the connections from straight through to crossover if they detect they are connected to another MDI.  In addition most new switches also have automatic crossover for auto MDI-X and auto-uplink which allows connections to be made with a straight through cable.

When trying to identify if you have a MDI, MDI-X issue with your network connection the wire map functionality of a cable or connectivity tester should be used to identify if the connection link or patch cable is a straight through or crossover cable and then determine if it is the correct correction for the connection of the two devices involve as described above.