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Extended unique identifiers

The IEEE administers the assignment of 24-bit identifiers, The identifier is formally known as an "Organizationally Unique Identifier" (OUI) or "company_id" when referring to octet-string or binary-number formats respectively. The phrase "OUI/company_id" refers to an identifier in either of these two formats.

The MAC-48 is a concatenation of the 24-bit OUI assigned by the IEEE Registration Authority and a 24-bit extension identifier assigned by the organization with that OUI assignment.

The EUI-48 is a concatenation of the 24-bit company_id OUI value assigned by the IEEE Registration Authority and a 24-bit extension identifier assigned by the organization with that company_id OUI assignment.

The EUI-64 is a concatenation of the 24-bit company_id OUI value assigned by the IEEE Registration Authority and a 40-bit extension identifier assigned by the organization with that company_id OUI assignment.

The EUI-48 and EUI-64 values are intended to be used within applications that require fixed size universal identifiers. Other applications may elect to use variable length identifiers (such as those defined in ASN.1, which support a more flexible and extensible hierarchy of organizational identifiers).

IEEE administered identifiers

The IEEE administers the assignment of 24-bit company_id OUI values. The assignments of these values are public, so that a user of a MAC-48, EUI-48, or EUI-64 value can identify the manufacturer that provided any value[1]. The IEEE/RAC has no control over the assignments of the extension identifiers and assumes no liability for assignments of duplicate MAC-48/EUI-48/EUI-64 identifiers assigned by manufacturers.

The 24-bit OUI/company_id value is intended to identify the organization that administers the remaining bits in EUI-48 and EUI-64 values. The OUI/company_id value should not be used (in isolation) to identify a vendor or the format of vendor-dependent information. When necessary to identify the vendor of a hardware device, an EUI-48 (or EUI-64)  identifier should be used. This allows large organizations to assign distinct EUI-48 identifiers, so that each division can be identified as a distinct "vendor". Alternatively, groups within an SDO (standards development organization) can be identified by distinct EUI-48 (or EUI-64) identifiers administered by their sponsoring body.

Identifier applications

In new applications, the OUI/company_id values are expected to generate extended unique identifiers of the following forms used to generate extended unique identifiers of the following forms:

NOTE: When used to identify a hardware instance in new applications, the IEEE/RAC intends to migrate from MAC-48  to EUI-64 identifiers. However, for backward compatibility, this transition be difficult for some 802-related applications. Therefore, policies for allowing selective use of 48-bit identifiers within 802-related systems are being developed (see the following subclause for further details).

The EUI-64 value was originally conceived as a mechanism to avoid excess consumption of OUI/company_id values within high-volume non-networking applications. Given the minimal probability of consuming all the EUI-64 identifiers, the IEEE/RAC places minimal restrictions on their use within standards.

The terms EUI-48 and EUI-64 are trademarked by the IEEE. Companies are allowed to use this term for commercial purposes, but only if their use of this term has been reviewed by the IEEE/RAC and the proposed products using the EUI-48 or EUI-64 conform to these restrictions.

MAC-48 identifier restrictions

The 48-bit MAC-48 identifiers were originally created to serve as network or MAC (Media Access Control) addresses for LANs (Local Area Networks) by IEEE Project 802. Within this environment,  MAC-48 identifiers are intended to identify items of real physical equipment, parts of such equipment, or functions that apply to many instances of physical equipment.

The use of 48-bit identifiers has been extended to serve as protocol identifiers to identify protocol designs and design revisions of protocols operating between instances of physical equipment; the name EUI-48 refers to identifiers use in this fashion. There are expected to be far fewer such protocols identified than there are items of addressable physical equipment.

The total number of MAC-48 identifiers available, while large, is NOT inexhaustible. The IEEE Registration Authority Committee (IEEE-RAC) has the duty to promote the continued availability of the MAC-48 capability in conjunction with IEEE standards and non-IEEE standards, for the benefit of the world-wide community using those standards.

With the exception of such protocol identifiers, MAC-48 identifiers are intended to identify items of real physical equipment or parts of such equipment such as separable subsystems or individually addressable ports. The expected use should not exceed one MAC-48 identifier per hardware subsystem or a most a very low number of MAC-48 identifier per physical instances of such equipment (e.g. groups of ports as in IEEE Std 802.3ad, for link aggregation). Allocation of a single MAC-48 bit identifier to identify or permit addressing of a fixed and permanent function associated with a real item of physical equipment occurs for the lifetime of that equipment or an indefinite period of use.

In particular any application that called for subdivision of the available number space, for block allocation to physical equipment without an identifiable physical instance per MAC-48 identifier, or for encoding functional capabilities within significant bits or bit patterns of the identifier, has the potential to rapidly exhaust the address space. To reduce the prospect of exhaustion, new applications and proposed extensions to current applications with significant volume expectations are STRONGLY encouraged to make use of EUI-64, rather than the MAC-48, to identify hardware instances.

New applications which require address format matching to the existing base of MAC-48 equipment will be reviewed by the IEEE-RAC and such exceptions will only be approved on a case-by-case basis. Given the widespread use of hardware identifiers, particularly within consumer based IP applications, the number of exceptions (which use MAC-48 versus EUI-64 identifiers to identify hardware instances) is expected to be small. Non-standard uses of MAC-48 are not supported.

The IEEE-RAC solicits any information that poses a threat to the viability of the unique MAC-48/EUI-48/EUI-64 address space, whether an IEEE proposed standard or another standard or specification. Further, in carrying out this duty to preserve the longevity of these identifier capabilities, the RAC will act, via liaison or direct coordination, to prevent potentially abusive uses for the consumption of the OUI.

The IEEE-RAC regards the consistent enforcement of these restrictions as a fundamental and realistic basis for ensuring longevity of the MAC-48 identifier capability, with a target lifetime of 100 years for existing applications using MAC-48 identifiers.

Nonoverlapping assignments

The organization that purchases an OUI/company_id is encouraged to assign only one form of MAC-48/EUI-48 identifier, regardless of application. Manufacturing entities within each organization can generate their EUI-64 assignments by appending an entity-dependent 16-bit sequence number to that code.

The IEEE/RAC recognizes that companies may have assigned distinct groups to administer MAC-48 and EUI-48 identifiers, and these groups (acting independently) may have generated matching MAC-48 and EUI-48 identifiers. This limited form of number-space overlap is discouraged but not forbidden.

However, duplication within each of these spaces is forbidden. For example, Thus, the EUI-48 values that specify I/O driver software interfaces, language codes, and hardware model numbers shall never overlap. Similarly, the EUI-64 values that specify I/O driver software interfaces, language codes, hardware model numbers, and hardware instances shall never overlap. This no-overlap strategy is expected to reduce unintentional duplication of EUI-48 values, by elimination of subjective application-class judgments, although a few more EUI-48 values may be consumed.

Identifier consumption

The manufacturer identifier assignment allows the assignee to generate approximately 1 trillion (1012) unique EUI-64 values, by varying the last 40 bits. Alternatively, the assignee can generate approximately 16 million (106) unique MAC-48/EUI-48 values, by varying the last 24 bits. The IEEE intends not to assign another OUI/company_id value to a manufacturer until the manufacturer has consumed, in product, the preponderance (more than 90%) of the block of potential MAC-48 or EUI-64 words. It is incumbent upon the manufacturer to ensure that large portions of the unique word block are not left unused in manufacturing.

Questions of use

Question: My company has consumed of the MAC-48 address space to distinctively identify networking cards in desktop computers. Another division is planning to produce laptop computers. Should that division apply for a different OUI/company_id identifier?

Answer: No. The organization is responsible for consuming a preponderance of its MAC-48 address space before requesting an additional OUI/company_id identifier. To avoid duplicate address assignments, a central authority should assign blocks of MAC-48 address space to each division, on a demand basis, until all blocks have been consumed.

Question: In my company, division A has consumed of the MAC-48 address space to distinctively identify networking cards. Now division B plans to produce IEEE Std 1394-1995 nodes, which have distinctive EUI-64 identifiers. Can we use the existing OUI/company_id identifier to generate these EUI-64 node identifiers?

Answer: Yes. The same OUI/company_id identifier can be used to generate these EUI-64 identifiers.

Question: Do the previously assigned MAC-48 addresses affect the assignment of future EUI-64 identifiers?

Answer: Previously consumed MAC-48 addresses have no effect on the assignment of future EUI-64 identifiers.

Question: My group is designing an IEEE Std 1394-1995 networking node, which has MAC-48 and EUI-64 identifiers in the 1394 and network interfaces respectively. Since both identifier effectively identify the same node, could the MAC-48 identifier be extended to form the EUI-64?

Answer: Yes. A valid (known to be unique) EUI-64 can be generated by encapsulating the MAC-48 identifier, as described in:
subclause Restricted and encapsulated values.

Question: I would like to distinctively identify software objects, or files stored on a disk drive. Can I use an EUI-64 identifier for this purpose?

Answer: No. When the volume of identifiers exceeds the one-per-hardware-instance volume, a larger identifier should be used. For example, files on a disk could be identified by concatenating an EUI-64 (that uniquely identifies the disk) with a 64-bit time stamp or sequence number.

[1]Except for private company_id OUI values, where the owner of the company_id OUI value is confidential. These shall remain private.





Copyright 2003 IEEE

(IEEE Standards Systems/Network Staff)
(Modified: 21-April 2003)

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