Organizationally Unique Identifier - Format

Format

The OUI is normally discussed and represented as a set of octets in hexadecimal notation separated by dashes (i.e., FF-FF-FF) or as a set of octets separated by colons in bit-reversed notation (i.e., FF:FF:FF).

The two least-significant-bits of the second nibble of the first octet of the hexadecimal representation (i.e., the two least significant bits of the first octet) of the OUI are reserved as flag bits for some protocols (e.g., 'M' bit and 'X' bit), flags to indicate whether the address is part of an individual (unicast) or group (multicast) address block (e.g., Individual / Group bit or Unicast / Multicast bit), flags to indicate whether an address is universally or locally administered (e.g., Universal / Local bit), etc., and should not contain the values 1,2,3,5,6,7,9,a, b,d, e, or f, unless these values reflect the true meaning of these flag bits – if the organization that owns the OUI does set one of these bits when creating an identifier, then the value of the second nibble of the first octet changes accordingly in representations of the OUI (e.g., if the hexadecimal value of the second nibble of the first octet is 'C' and the least-significant-bit is set, then the value becomes 'D').

Note: “Three-octet values occupying the same fields as OUIs can occupy, but with the next-to-LSB of the first octet set to 1, are locally assigned and have no relationship to the IEEE-assigned values...”.

Example:

The following figure shows the position of these bits in significance order:

| OUI | | Octet 0 | Octet 1 | Octet 2 | | nibble | nibble | nibble | | __||__ | __||__ | __||__ | | | | | | | | | | | | 0 || 1 | 2 || 3 | 4 || 5 | |bits||bits|bits||bits|bits||bits| |7654||3210|7654||3210|7654||3210| ||||| ||||||||| ||||||||| ||||| | A C | D E | 4 8 | |1010 1100|1101 1110|0100 1000| | | || | | | | || | least-significant-bit of OUI | | || least-significant-byte of OUI | | |least-significant-bit of first octet of OUI = I/G or M bit | | next-to-least-significant-bit of first octet of OUI = U/L or X bit | most-significant-byte of OUI most-significant-bit of OUI

Note: The OUI of AC-DE-48 is used for illustrative purposes only and this information applies regardless of the actual value of the OUI.

Notes:

1. The IEEE also refers to the OUI as 'company_id' when the OUI is written as a base 16 number (i.e., as a hexadecimal number with no dashes or colons separating the octets of the OUI).
2. In the following, 'F' and 'h' represent any hexadecimal number.
3. In the following, 'c' represents the digits of the 'company_id', and 'e' represents the digits of the extension identifier supplied by the organization to whom the OUI is registered.