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Networking Fundamentals · Foundational

Organizationally Unique Identifier (OUI)

The first 24 bits of a MAC address, assigned by the IEEE to identify the device manufacturer.

Definition

Vendors buy OUIs from the IEEE Registration Authority and assign the remaining 24 bits uniquely to each interface. OUI lookups help identify unknown devices in ARP tables and packet captures. Randomized MAC addresses set the locally administered bit and do not use a real OUI.

Example

A MAC beginning 00:50:56 identifies a VMware virtual NIC.

Exam tip

OUI = first half (vendor); second half = device-specific.

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