Final answer:
The Organizationally Unique Identifier (OUI) is the first six hexadecimal digits of the MAC address, which identify the manufacturer or organization that made the device.
Step-by-step explanation:
The Organizationally Unique Identifier (OUI) is a part of the MAC address used to uniquely identify a device on a network. The OUI is the first six hexadecimal digits of the MAC address and is assigned by the Institute of Electrical and Electronics Engineers (IEEE) to identify the manufacturer or organization that produced the network interface card or device. By having these first six digits, the OUI helps in categorizing and organizing devices globally. The MAC address itself is a 12-digit hexadecimal number, where the latter six digits (after the OUI) are assigned by the manufacturer and are unique to the specific device.