Organizationally Unique Identifier

An Organizationally Unique Identifier (OUI) is a 24-bit number assigned by the IEEE Registration Authority to manufacturers or vendors of network devices. These globally unique identifiers serve as the foundation for creating unique device addresses on computer networks.

OUIs are primarily used to construct MAC addresses, which uniquely identify network interfaces. A MAC address consists of 6 octets (48 bits total), where the first three octets form the OUI, identifying the manufacturer, and the last three octets represent the device-specific identifier assigned by that manufacturer.

OUI Structure and Format

A 3-octet OUI is typically represented in hexadecimal notation using different separators:

• Dash notation − 1F-9E-A0

• Colon notation − 1F:9E:A0

• Period notation − 1F9E.A0 (Cisco format)

How OUI Works

When the IEEE assigns an OUI to a manufacturer, that company receives exclusive rights to use those first 24 bits in their device MAC addresses. The manufacturer can then create up to 16,777,216 unique MAC addresses (2^24 combinations) by varying the last 24 bits.

Common OUI Examples

Applications of OUI

• Network device identification − Helps network administrators identify device manufacturers

• Security analysis − Used to detect unauthorized devices on networks

• Asset management − Assists in inventory tracking and device categorization

• Wake-on-LAN − Enables remote device activation using MAC addresses

Conclusion

The OUI system ensures global uniqueness of MAC addresses by providing manufacturers with exclusive 24-bit identifiers. This standardized approach enables reliable device identification and network management across diverse computing environments.