What is unique about a MAC address?

The correct answer highlights that a MAC address is indeed a unique identifier, but it isn't a 12-bit identification number; rather, it is a 48-bit identifier usually expressed in hexadecimal format. Each MAC address is assigned to a network interface card (NIC) by the manufacturer. The uniqueness is crucial because it allows devices to communicate on a network without confusion.

Among the other choices, while a MAC address can be changed by some users through software modifications, it is not primarily designed to be user-adjustable. It is tied to the hardware rather than the operating system. Lastly, although MAC addresses are unique to each device, they are not universally shared identifiers since they are specific to local area networks and are not routable over the Internet. The correct context is essential to understanding the role MAC addresses play in networking.