专用媒体接口（Medium Dependent Interface），简称MDI。普通主机、路由器等的网上接口类型通常为MDI，在许多集线器和交换机上都有一个uplink端口，用于连接到另一个集线器或交换机，这个端口通常也采用MDI类型。

A medium dependent interface (MDI) describes the interface (both physical and electrical) in a computer network from a physical layer implementation to the physical medium used to carry the transmission. Ethernet over twisted pair also defines a medium dependent interface crossover (MDI-X) interface. Auto MDI-X ports on newer network interfaces detect if the connection would require a crossover, and automatically chooses the MDI or MDI-X configuration to properly match the other end of the link.