Organizationally Unique Identifier

An organizationally unique identifier (OUI) is a 24-bit number that uniquely identifies a vendor, manufacturer, or other organization.

These are purchased from the Institute of Electrical and Electronics Engineers, Incorporated (IEEE) Registration Authority by the "assignee" (IEEE term for the vendor, manufacturer, or other organization). They are used as the first portion of derivative identifiers to uniquely identify a particular piece of equipment as MAC addresses, protocol identifiers, World Wide Names for Fibre Channel host bus adapters, and other Fibre Channel and Serial Attached SCSI devices.

In MAC addresses, the OUI is combined with a 24-bit number (assigned by the owner or 'assignee' of the OUI) to form the address. The first three octets of the address are the OUI.

The following terms are defined (either implicitly or explicitly) in IEEE Standard 802-2001 for use in referring to the various representations and formats of OUIs and the identifiers that may be created using them.

“The representation of a sequence of octet values in which the values of the individual octets are displayed in order from left to right, with each octet value represented as a two-digit hexadecimal numeral, and with the resulting pairs of hexadecimal digits separated by hyphens. The order of the hexadecimal digits in each pair, and the mapping between the hexadecimal digits and the bits of the octet value, are derived by interpreting the bits of the octet value as a binary numeral using the normal mathematical rules for digit significance.” (See hexadecimal).

“The format of a MAC data frame in which the octets of any MAC addresses conveyed in the MAC user data field have the same bit ordering as in the Hexadecimal Representation.” (See MAC data frame, MAC addresses)

This appears from the context of the IEEE Standard 802-2001 to be another term for the 'Hexadecimal Representation' – i.e., “by interpreting the bits of the octet value as a binary numeral using the normal mathematical rules for digit significance.”

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