What is the primary purpose of the MAC address in the data link layer?

• A. It determines the IP address of the device.
• B. It uniquely identifies a network interface on a local segment for frame delivery and forwarding.
• C. It governs how many hops a frame can traverse.
• D. It encrypts frames for secure transmission.

Answer: (D)

MAC addresses function as unique identifiers for network interfaces on a local link, and they are what allows a frame to be delivered to the exact device within the same broadcast domain. In the data link layer, every frame carries a destination MAC and a source MAC. Switches learn which devices (MAC addresses) are on which ports and use that information to forward frames to the correct port, ensuring the frame reaches the intended NIC on the local network.

IP addressing operates at the network layer and is about reaching devices across different networks, not about delivering frames on the local segment. The hop limit (how many routers a frame can traverse) is controlled by layer-3 mechanisms like TTL, not by MAC addresses. Encryption is a security concern handled by other protocols and cryptographic methods, not by the MAC address itself.