Which of the following is NOT true about MAC addresses?

The statement that MAC addresses can be changed at will is not true in the sense that while it is technically possible to change or "spoof" a MAC address through specific software configurations or network settings, this is not the inherent function or characteristic of a MAC address. MAC addresses are designed to provide a unique identifier for network interfaces at the data link layer, ensuring proper communication within a local network environment.

In contrast, the other statements accurately reflect the nature of MAC addresses. They are indeed unique to each network interface, ensuring that devices can be distinctly identified on a network. MAC addresses consist of 48 bits, typically represented in hexadecimal format, and their primary function is performed at the data link layer of the OSI model, which is responsible for node-to-node data transfer and error detection/correction.