Medium dependent interface
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A medium dependent interface is also referred to as an MDI port or an uplink port. Abbreviated MDI, an Ethernet port connection typically used on the Network Interface Card (NIC) or Integrated NIC port on a PC. Since inputs on an NIC must go to outputs on the router, hub or switch, these latter devices have their inputs and outputs (transmit and receive signals) reversed in a configuration known as MDIX or MDI-X. Some network hubs or switches have a MDI port (often switchable) in order to connect to other hubs or switches without a null-modem, or Ethernet crossover cable.
Auto MDI/MDIX ports on newer hubs or switches are designed to detect if the connection is backwards and automatically chooses MDI or MDIX to properly match the connection.
For a diagram of non-automatic connections between MDI and MDIX devices, see donbot.com
