An organizationally unique identifier (OUI) is a 24-bit number that uniquely identifies a vendor, manufacturer, or other organization.
OUIs are purchased from the Institute of Electrical and Electronics Engineers (IEEE) Registration Authority by the assignee (IEEE term for the vendor, manufacturer, or other organization). They are used to uniquely identify a particular piece of equipment through derived identifiers such as MAC addresses, Subnetwork Access Protocol protocol identifiers, World Wide Names for Fibre Channel devices.
In MAC addresses, the OUI is combined with a 24-bit number (assigned by the assignee of the OUI) to form the address. The first three octets of the address are the OUI.