What uniquely identifies a device on a network and is unable to be changed?

A MAC address, or Media Access Control address, is a unique identifier assigned to network interfaces for communications on the physical network segment. It serves as a unique signature for each network device, ensuring that data packets are directed to the correct hardware on a local area network (LAN).

MAC addresses are hard-coded into the hardware by the manufacturer and are generally unable to be changed without specialized tools or methods. This permanence makes them essential for identifying devices distinctly on a network. Each MAC address consists of six groups of two hexadecimal digits, and no two devices should have the same MAC address on the same network, ensuring reliable communication.

In contrast, an IP address can change due to various reasons such as device relocation to a different network or using DHCP (Dynamic Host Configuration Protocol) to assign addresses dynamically. Similarly, a network ID is associated with a broader range of devices on the same network and can also change over time. A host name is a user-friendly label for a device on a network and can be modified at will.

Thus, the MAC address is the only choice that uniquely identifies a specific device on a network and remains unchanged, making it the correct answer.