Organizationally Unique Identifiers (OUIs) are the foundation of MAC address vendor identification. This guide explains what OUIs are, how they work, and how to use them to identify network device manufacturers.

What is an OUI?

An OUI (Organizationally Unique Identifier) is a 24-bit number that uniquely identifies a vendor, manufacturer, or organization that makes network devices. It forms the first half of a MAC address, which allows network administrators to identify the maker of a device by examining its MAC address.

OUIs are assigned by the IEEE Registration Authority (IEEE-RA) to ensure uniqueness across all network devices manufactured globally. Once a manufacturer is assigned an OUI, they can use it to create millions of unique MAC addresses for their devices.

OUI Format and Structure

An OUI consists of the first 24 bits (3 bytes) of a MAC address. It is typically represented as 6 hexadecimal digits. For example, in the MAC address 00:1A:2B:3C:4D:5E, the OUI is 00:1A:2B.

Special Bits in the OUI

Within the OUI, two bits have special significance:

1. Universal/Local (U/L) Bit - The second least significant bit (bit 1) of the first byte:
   • 0 = Universally administered (assigned by manufacturer)
   • 1 = Locally administered (assigned by network administrator)
2. Individual/Group (I/G) Bit - The least significant bit (bit 0) of the first byte:
   • 0 = Individual address (unicast)
   • 1 = Group address (multicast)

Because of these special bits, manufacturers are typically assigned OUIs with even values for the first byte (e.g., 00, 02, 04, etc.), ensuring that by default, addresses are universally administered and unicast.

How to Perform an OUI Lookup

To determine the manufacturer of a network device, you can look up its OUI in the IEEE's registry or use an OUI lookup tool like MACVerify. Here's the process:

1. Extract the OUI - Take the first 6 digits (3 bytes) of the MAC address
2. Format the OUI - Convert to a consistent format (e.g., 00:1A:2B)
3. Perform the lookup - Search for the OUI in a database or lookup tool
4. Identify the vendor - Match the OUI to the corresponding manufacturer

Example OUI Lookup

Let's say you have a device with MAC address 00:1A:2B:3C:4D:5E:

1. Extract the OUI: 00:1A:2B
2. Use our MAC address lookup tool to find the manufacturer
3. The result will show the vendor associated with this OUI

The IEEE OUI Registry

The IEEE Registration Authority maintains the official registry of OUIs. This database contains all assigned OUIs along with their corresponding organization names and addresses. The registry is regularly updated as new OUIs are assigned to organizations.

The IEEE provides this information in several formats:

• OUI Public Listing - A text file containing all assigned OUIs
• MA-L (MAC Address Block Large) - Previously known as OUI, assigns 24 bits
• MA-M (MAC Address Block Medium) - Assigns 28 bits
• MA-S (MAC Address Block Small) - Assigns 36 bits

MACVerify maintains an up-to-date database derived from the IEEE's registry, containing over 74,836 entries.

Common OUI Assignments

Here are some well-known manufacturers and their OUI assignments:

OUI	Manufacturer	Common Devices
00:1A:11	Google, Inc.	Chromecast, Google Home, Nest devices
00:03:93	Apple, Inc.	Older Mac computers
00:05:02	Apple, Inc.	Older Apple devices
00:0C:29	VMware, Inc.	Virtual machines
00:13:A9	Sony Corporation	PlayStation, Bravia TVs
00:18:0A	Cisco Systems, Inc.	Routers, switches, access points
08:00:27	PCS Systemtechnik GmbH	Oracle VirtualBox VMs
B8:27:EB	Raspberry Pi Foundation	Raspberry Pi devices

You can explore our complete vendor directory to see all manufacturers and their assigned OUIs.

Limitations of OUI Lookup

While OUI lookup is a powerful tool for network administrators, it does have some limitations:

• MAC Spoofing - MAC addresses can be changed through software, making OUI identification unreliable if spoofing is involved
• Company Acquisitions - When one company acquires another, their products may continue to use the OUIs assigned to the acquired company
• Multiple OUIs - Large manufacturers often have multiple OUIs assigned to them, which can make identification challenging
• Locally Administered Addresses - If the U/L bit is set (indicating a locally administered address), the OUI does not reliably identify the manufacturer
• Virtualization - Virtual machines often use software-generated MAC addresses that may not correspond to actual hardware vendors

Using MACVerify for OUI Lookup

Our MAC address lookup tool simplifies the OUI lookup process by automatically:

1. Extracting the OUI from any MAC address format you enter
2. Searching our comprehensive database of 74,836 entries
3. Displaying the manufacturer information instantly
4. Providing additional details about the MAC address structure

Simply enter a full or partial MAC address in the search box, and our tool will identify the manufacturer and provide detailed information about the device.

Conclusion

OUI lookup is an essential technique for network administrators and security professionals to identify device manufacturers on their networks. By understanding how OUIs work and using tools like MACVerify, you can quickly determine the origin of devices connecting to your network.

For more information about MAC addresses and networking concepts, explore our other resources:

• What is a MAC Address?
• MAC Address Format Guide
• MAC Address Types
• Finding Your MAC Address
• MAC Address Security