What is the characteristic of a MAC address?

A MAC (Media Access Control) address is indeed a 48-bit number that is assigned to network interfaces for communications on the physical network segment. It serves as a unique identifier for devices within a network. The significance of its 48-bit structure is that it allows for a vast number of unique addresses, enabling distinct identification for numerous devices.

While MAC addresses are generally considered globally unique due to the way they are assigned by manufacturers—who are allocated specific ranges of addresses—it's important to note that they can also be altered or spoofed. Some network devices allow users to change their MAC addresses for privacy or security reasons, which adds to the versatility of MAC addresses but also introduces potential security vulnerabilities.

In contrast, the other options describe characteristics that do not align with the nature of a MAC address. A 64-bit unique identifier assigned to users is not reflective of MAC addresses, as they have a specific 48-bit format. An identification mechanism linked to user authentication pertains more to user credentials and access control methods rather than device identification. Lastly, a strong password used for device security is also distinctly separate from what a MAC address represents, as passwords are utilized for user authentication and protecting access, rather than identifying network interfaces.