What is an Organizationally Unique Identifier (OUI)?

Published on September 30, 2025

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OUI in the device identity and standards context

An Organizationally Unique Identifier (OUI) is a 24-bit number. Regulators use it to show the organization. That group assigns the remaining identifier bits to network hardware.

The IEEE Registration Authority assigns the OUI. It helps create unique identifiers used in IEEE 802 standards.

For example, manufacturers often embed OUIs in hardware. They can appear as the first three octets of interface addresses in Ethernet and other networks.

Definition of OUI

From the standards view, an OUI is a 24-bit number. The IEEE Registration Authority assigns or sells it. It uniquely identifies a vendor, manufacturer, or other organization.

Note: some vendors call the OUI a ‘company_id’ in their documents. Each OUI is meant to be globally unique, according to IEEE records.

The OUI provides a vendor-level identifier. It combines with other bits from the organization to create an EUI-48 (extended unique identifier-48) or EUI-64 identifier.

Structure of an OUI

By definition, the OUI takes the first three octets of a 48-bit identifier, like an EUI-48 or MAC-48 (medium access control address-48). It is shown in hexadecimal format.

The last three octets of a 48-bit identifier are assigned by the OUI owner. They identify individual interfaces made by that organization.

For example, the standard form often shows hex octets separated by dashes (AC-DE-48) or colons. Some transmission formats use a bit-reversed order for sending over wires.

In protocols, two low-order bits in the first octet act as flags. These bits, like U/L (Universal/Local) and I/G (Individual/Group), can change how the address is interpreted or used.

Assignment and Registration

The IEEE Registration Authority issues OUIs. They are listed in registries, like the MA-L (MAC Address Block Large) product, which documents the assignments.

Note: organizations get OUIs through IEEE procedures. This usually gives them a permanent 24-bit value. They can use it in different identifier formats.

For clarity, related registry products include MA-L, MA-M (MAC Address Block Medium), and MA-S (MAC Address Block Small). They offer different block sizes and uses. IEEE registration materials explain these differences.

The MA-L registry is the main source for three-octet OUI assignments. Other registries follow different allocation rules and sizes.

Applications of OUI

OUIs are used to create Medium Access Control (MAC) and Extended Unique Identifier (EUI) values. These values show which network interfaces and devices are which. They work in Ethernet, Wi-Fi, and other IEEE 802 systems.

OUIs are also used in other systems. Examples include subnetwork access protocol (SNAP) company IDs, Fibre Channel World Wide Names, and extended display identification data (EDID) vendor blocks for displays.

For example, an OUI combined with vendor-assigned bits creates an EUI-48. An OUI with a larger extension can create an EUI-64, used in IPv6 addresses.

Manufacturers can program OUIs into devices during production. This gives each device a vendor-traceable identifier, following IEEE rules.

Examples

In practice, a vendor OUI shows up as the first three octets of a 6-octet address. For example, in 00-1B-32:xx-xx-xx, 00-1B-32 is the OUI.

For example, vendor documents may list company OUIs for servers, host bus adapters (HBAs), or switches. These lists are often used in support materials.

Administrators can check the IEEE OUI database or public lookup tools. This helps them match the three-octet prefix to the registered owner and verify the device’s origin.

Importance of OUI

OUIs help identify device vendors. This makes it easier to track inventory and manage support tasks based on the manufacturer.

Note: OUIs help with interoperability checks. Standardized assignments reduce identifier conflicts between manufacturers when used according to IEEE rules.

Using OUIs can make troubleshooting easier. Investigators can use an interface address to find the vendor. They can also identify the product family.

OUI and Network Security

From a security view, OUI information can help control access. Policies can allow or block devices based on their manufacturer, following set procedures.

OUIs can help with intrusion detection. They show if a device comes from an expected vendor, matching the known network baseline.

An OUI alone does not prove a device is safe. But it can flag unusual vendor prefixes. It can also support other evidence during investigations.

Related Identifiers and Variants

From a standards history view, several identifiers come from or extend OUIs. These include MAC-48 (historical), EUI-48, and EUI-64. IEEE is in charge of documenting all of it.

Note: IEEE now prefers the term EUI-48 instead of the old MAC-48. EUI-64 is used for longer identifiers, such as those in some IPv6 address methods.

Older identifiers include company IDs (CDI-32 and CDI-40). Registry items include Individual Address Block (IAB) and OUI-36. These are mentioned in IEEE documents. Current registries have replaced or deprecated many of these.

Representation and Formatting Issues

In formatting, hexadecimal display shows octets from left to right. Some transmission formats use bit-reversed or nonstandard order, which can cause apparent differences.

In token ring and some older standards, bits were sent in a different order. This can create different text representations if not converted to the standard format.

To interpret OUIs accurately, you must know the representation rules. You also need to understand bit-ordering, as described in IEEE references. This prevents misidentifying an OUI.

Registry Changes and Deprecations

As for administration, IEEE has made some changes. It no longer prefers the term MAC-48. IAB and OUI-36 activities moved to the newer MA-S registry on certain policy dates.

Note: Standards used methods that converted MAC-48 to EUI-64 in the past. IEEE now recommends using EUI formats directly and deprecates the old methods.

Old registry products are still relevant for legacy records. Today, current practice follows the active registries maintained by the IEEE Registration Authority.

Using OUI Data as Technical Evidence in Expert Testimony

From an expert witness’s view, OUI data can provide objective technical details. It shows vendor information, device counts, and compliance with IEEE records.

When preparing testimony, experts can use IEEE registry entries or device build documents. This supports technical findings that are factual and verifiable.

OUI-based mappings are one piece of evidence. They should be presented as technical observations. These observations must follow standard inspection methods and documents.

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Frequently Asked Questions About Organizationally Unique Identifiers (OUIs)

How can I look up the manufacturer of a device using its OUI?

You can check the first three octets of a 48-bit address. Use IEEE registries or vendor lookup tools for this. This helps identify the registered owner.

What is the difference between an OUI, MAC address, and EUI-48?

Note: An OUI is a 24-bit vendor prefix. An EUI-48 is the full 48-bit identifier, made by adding a 24-bit extension from the vendor. MAC is an older term used for this identifier.

Can two manufacturers share the same Organizationally Unique Identifier (OUI)?

From a registry policy view, each OUI goes to one assignee. Registry rules do not give the same 24-bit value to more than one organization.

Why are OUIs important for network security?

OUIs show the device vendor. This helps check where a device came from. You can also use them to create manufacturer-based policies. These policies follow security and management rules.

Are Organizationally Unique Identifiers (OUIs) ever reused or do they expire?

From a registry view, assigned OUIs stay linked to the owner in IEEE records. Some registry products are now deprecated. But the assigned values remain in the registration history.

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