Which statement correctly identifies the role of the first three bytes of a MAC address?

• A. Organizationally Unique Identifier (OUI) can be used to identify the NIC vendor
• B. The last three bytes identify the NIC vendor
• C. The first six bytes identify the NIC vendor
• D. The entire MAC address identifies the NIC vendor

Answer: (A)

The key idea is how a MAC address is built. It is six bytes long, with the first three bytes forming the Organizationally Unique Identifier (OUI) assigned by IEEE to a specific vendor. That OUI is what lets you identify the NIC vendor. The remaining three bytes are a device identifier chosen by that vendor to distinguish individual NICs. So, knowing the OUI tells you which company made the NIC, while the full six-byte address uniquely identifies a particular NIC. The other statements don’t fit because the vendor is not identified by the last three bytes, and saying the entire MAC address identifies the vendor overlooks that the vendor information comes from the OUI, not the complete address.