Organizationally unique identifier

“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.”[4] (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.”[4] (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.”[4]

“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 colons. 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 reversing the order of the bits in the octet value and interpreting the resulting bit sequence as a binary numeral using the normal mathematical rules for digit significance.”[4]

The bit-reversed representation corresponds to the convention of transmitting the least-significant-bit of each byte first in serial data communications.

“The format of a MAC data frame in which the octets of MAC addresses conveyed in the MAC user data field have the same bit ordering as in the Bit-reversed representation.”[4]

The order in which an octet or a sequence of octets is transmitted over the transmission medium – this order normally corresponds to the bit-reversed representation.

Example:

An OUI consisting of the hexadecimal digits ACDE48₁₆ would be represented as follows:

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

Notes:

1. The OUI of AC-DE-48 could be in use and is not a reserved value.
2. 'F' and 'h' represent any hexadecimal number.
3. 'c' represents the digits of the OUI, and 'e' represents the digits of the extension identifier supplied by the organization to whom the OUI is registered.

Ethernet users are used to seeing canonical form, such as in the output of the ifconfig command. Canonical form is the intended standard.

However, since IEEE 802.3 (Ethernet) and IEEE 802.4 (Token Bus) send the bytes (octets) over the wire, left-to-right, with least significant bit in each byte first, while IEEE 802.5 (Token Ring) and IEEE 802.6 (FDDI) send the bytes over the wire with the most significant bit first, confusion may arise when an OUI in the latter scenario is represented with bits reversed from the canonical representation. So for instance, an OUI whose canonical form is ACDE48 could be seen written as 357B12 if translation is done improperly or inconsistently. The latter form (bit-reversed or noncanonical representation), may also be referred to in the literature as "MSB format", "IBM format", or "Token Ring format" for this reason. RFC2469 explains the problem in more detail.

The CDI-32 was historically recommended as context dependent identifier that was formed by concatenating the 24-bit OUI with an 8-bit extension identifier that is assigned by the organization that purchased the OUI – the resulting identifier was generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF or FF:FF:FF:FF, as a string of 4 bytes as in {FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFF₁₆.[5]

The CDI-40 was historically recommended as context dependent identifier that was formed by concatenating the 24-bit OUI with a 16-bit extension or by concatenating a 36-bit OUI-36 with a 4-bit extension.[5] In either case, the extension was assigned by the organization that purchased the OUI. The resulting identifier was generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF-FF or FF:FF:FF:FF:FF, as a string of 5 bytes as in {FF,FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFFFF₁₆.

Note: There were also IAB based CDI-40 sequences that were formed by combining the 36-bit IEEE assigned IAB base value with the 4-bit extension identifier assigned by the organization – e.g., if the IEEE assigned IAB base value is 0x0050C257A and the 4-bit extension identifier is 0xF, then the CDI-40 values generated by combining these two numbers are from 0x0050C257AF00 to 0x0050C257AFFF

The IEEE now considers the label MAC-48 to be an obsolete term which was previously used to refer to a specific type of EUI-48 identifier used to address hardware interfaces (e.g., Network Interface Controllers and other network hardware) within existing IEEE 802 based networking applications and should not be used in the future.[5] Instead, the term EUI-48 should be used by manufacturers and others in the field for this purpose – i.e., MAC-48 identifier is identical to the EUI-48 identifier and is an obsolete label for it, although some distinction is still made when encapsulating MAC-48 and EUI-48 identifiers within EUI-64 identifiers (but now, the encapsulating mechanism is also deprecated).[5]

The EUI-48 is an identifier that is formed by concatenating the 24-bit OUI with a 24-bit extension identifier that is assigned by the organization that purchased the OUI – the resulting identifier is generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF-FF-FF or FF:FF:FF:FF:FF:FF, as a string of 6 bytes as in {FF,FF,FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFFFFFF₁₆.[5]

The EUI-60 is an identifier that is formed by concatenating the 24-bit OUI with a 36-bit extension identifier that is assigned by the organization that purchased the OUI – the resulting identifier is generally represented by a string of 15 nibbles, as a base 16 number as in FFFFFFFFFFFFFFF₁₆, or as FF-FF-FF:F.F.F.F.F.F.F.F.F as an EUI-64 value.[5]

Note: This identifier was previously used as the worldwide name (WWN) identifier within some storage systems. Its use is now considered deprecated by the IEEE and the EUI-64 identifier should be used in the future for this and all other purposes for which the EUI-60 was previously used. Some of the storage systems in which an OUI based variant was used are Fibre Channel, and Serial Attached SCSI (SAS).[5]

The EUI-64 is an identifier that is formed by concatenating the 24-bit OUI with a 40-bit extension identifier that is assigned by the organization that purchased the OUI – the resulting identifier is generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF-FF-FF-FF-FF or FF:FF:FF:FF:FF:FF:FF:FF, as a string of 8 bytes as in {FF,FF,FF,FF,FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFFFFFFFFFF₁₆.[5]

Note: According to the IEEE guidelines, the first four digits of the organizationally assigned identifier (i.e., the first four digits of the extension identifier) portion of an EUI-64 “shall not be FFFE₁₆ or FFFF₁₆” (i.e., EUI-64 identifiers of the form ccccccFFFEeeeeee and ccccccFFFFeeeeee are not allowed) – this is to support the encapsulation of EUI-48 (FFFE₁₆) and MAC-48 (FFFF₁₆) values into EUI-64 values (though now the encapsulation is deprecated).

IPv6 uses a 64-bit Modified Extended Unique Identifier (Modified EUI-64) in the lower half of some IPv6 addresses. A Modified EUI-64 is an EUI-64 with the U/L bit inverted.[6]

There are other identifiers that may be formed using the OUI but those listed above are the most commonly used.

Encapsulation of MAC-48 within EUI-64 Example:

Assuming that an organization has registered the OUI of AC-DE-48 and that the organization has created the MAC-48 value of AC-DE-48-23-45-67 by concatenating the extension identifier 23-45-67, this MAC-48 identifier has the following binary transmission order:

 |            OUI             |     extension identifier   | field
 |  1st   |   2nd   |  3rd    |   4th   |   5th   |   6th  | octet
 | C   A  |  E   D  |  8   4  |  3   2  |  5   4  |  7   6 | hex
 0011 0101 0111 1011 0001 0010 1100 0100 1010 0010 1110 0110 bits
 |       | |       | |       | |       | |       | |       |
 lsb   msb lsb   msb lsb   msb lsb   msb lsb   msb lsb   msb

The same MAC-48 identifier after encapsulation within an EUI-64 has the following transmission order:

 |           OUI           |    MAC label    |   extension identifier  | field
 |  1st  |  2nd   |  3rd   |  4th   |  5th   |  6th   |  7th   |  8th  | order
 | C  A  |  E  D  |  8  4  |  F  F  |  F  F  |  3  2  |  5  4  |  7  6 | hex
 00110101 01111011 00010010 11111111 11111111 11000100 10100100 11100110 bits
 |      | |      | |      | |      | |      | |      | |      | |      |
 lsb  msb lsb  msb lsb  msb lsb  msb lsb  msb lsb  msb lsb  msb lsb  msb

The same MAC-48 identifier after encapsulation within an EUI-64 has the following significance order:

 |           OUI           |    MAC label    |   extension identifier  | field
 |  AC   |   DE   |   48   |   FF   |   FF   |   23   |   45   |   67  | hex
 10101100 11011110 01001000 11111111 11111111 00100011 01000101 01100111 bits
 |  |                                                               |  |
 |  most-significant-byte                      least-significant-byte  |
 most-significant-bit                              least-significant-bit

Encapsulation of EUI-48 within EUI-64 example:

Assuming that an organization has registered the OUI of AC-DE-48 and that the organization has created the EUI-48 value of AC-DE-48-23-45-67 by concatenating the extension identifier 23-45-67, this EUI-48 identifier has the following format in significance order:

 |        company_id       |   extension identifier  | field
 |  AC   |   DE   |   48   |   23   |   45   |   67  | hex
 10101100 11011110 01001000 00100011 01000101 01100111 bits
 |  |                                             |  |
 |  most-significant-byte    least-significant-byte  |
 most-significant-bit            least-significant-bit

The same EUI-48 identifier after encapsulation within an EUI-64 has the following format in significance order:

 |        company_id       |    EUI label    |   extension identifier  | field
 |  AC   |   DE   |   48   |   FF   |   FE   |   23   |   45   |   67  | hex
 10101100 11011110 01001000 11111111 11111110 00100011 01000101 01100111 bits
 |  |                                                               |  |
 |  most-significant-byte                      least-significant-byte  |
 most-significant-bit                              least-significant-bit

Encapsulation of MAC-48 or EUI-48 within modified EUI-64 example:

In the encapsulation within a Modified EUI-64 a MAC-48 is treated as an EUI-48 and the U/L bit is inverted.[6] Assuming that an organization has registered the OUI of AC-DE-48 and that the organization has created the MAC-48 or EUI-48 value of AC-DE-48-23-45-67 by concatenating the extension identifier 23-45-67, this MAC-48 or EUI-48 identifier has the following format in significance order:

 |        company_id       |   extension identifier  | field
 |  AC   |   DE   |   48   |   23   |   45   |   67  | hex
 10101100 11011110 01001000 00100011 01000101 01100111 bits
 |  |                                             |  |
 |  most-significant-byte    least-significant-byte  |
 most-significant-bit            least-significant-bit

The same MAC-48 or EUI-48 identifier after encapsulation within a Modified EUI-64 has the following format in significance order:

 |        company_id       |    EUI label    |   extension identifier  | field
 |  AE   |   DE   |   48   |   FF   |   FE   |   23   |   45   |   67  | hex
 10101110 11011110 01001000 11111111 11111110 00100011 01000101 01100111 bits
 |  |                                                               |  |
 |  most-significant-byte                      least-significant-byte  |
 most-significant-bit                              least-significant-bit

Network Address Authority (NAA) Name_Identifier formats define the first nibble (4 bits) to define the format of the identifier:

This encapsulation is used in Fibre Channel[7] and SAS, and is also supported in iSCSI in RFC 3980. This addition requires either a shortened vendor-specific identifier field, or some OUI bits are assumed to be 0, such as when using EUI-64 Mapped format.