In an IEEE 48-bit MAC address, which portion is known as the organizationally unique identifier (OUI)?

Introduction / Context:
MAC addresses are 48-bit hardware addresses used at the Data Link layer to uniquely identify network interface cards on Ethernet and other LAN technologies. To ensure global uniqueness, IEEE allocates blocks of MAC addresses to manufacturers. The organizationally unique identifier (OUI) is the portion of the MAC address that identifies the vendor or organization that manufactured the device.

Given Data / Assumptions:

A standard MAC address is 48 bits long, usually written as six groups of two hexadecimal digits (for example, 00:1A:2B:3C:4D:5E).
The OUI identifies the vendor and is assigned by the IEEE Registration Authority.
We need to determine which part of the 48-bit address constitutes the OUI.

Concept / Approach:
The MAC address can be split into two 24-bit halves. The first 24 bits (the first three bytes) are the OUI, which is unique to a manufacturer or organization. The remaining 24 bits (the last three bytes) are assigned by the vendor to identify individual interfaces or devices. This scheme allows a large number of unique MAC addresses to be generated under each OUI.

Step-by-Step Solution:
Write a MAC address in the common format: aa:bb:cc:dd:ee:ff. The first three octets (aa, bb, cc) together form 24 bits, known as the organizationally unique identifier (OUI). The last three octets (dd, ee, ff) form the device specific portion, assigned by the vendor to individual NICs. Therefore, the OUI is the first 24 bits (3 bytes) of the address. Among the options, only option a correctly describes this structure.

Verification / Alternative check:
If you search for OUI databases or vendor lookup tools, they typically accept the first three bytes of a MAC address (for example 00-1A-2B) to identify the manufacturer. This practical usage confirms that the OUI is exactly the first 24 bits of the 48-bit MAC address and not any other portion.

Why Other Options Are Wrong:
Option b mentions the first 12 bits but then incorrectly states "3 bytes"; 12 bits is only 1.5 bytes and does not match the actual OUI size.
Option c says the first 24 bits or 6 bytes, which is inconsistent; 6 bytes would be the entire MAC address, not just the OUI.
Option d refers to the first 32 bits, which would be 4 bytes, not 3, and is not how OUIs are defined.
Option e incorrectly identifies the last 24 bits as the OUI; these bits are vendor assigned device identifiers, not the organizational identifier.

Common Pitfalls:
Students sometimes misinterpret the term "organizationally unique" and assume it refers to the entire MAC address. In reality, uniqueness is ensured by combining a unique OUI with a vendor assigned device portion. Another pitfall is confusing MAC addresses with IPv6 interface identifiers, which have different formats and structures.

Final Answer:
The OUI is the first 24 bits, or 3 bytes, of the MAC address.