Apple Certified Support Professional (ACSP) Practice Exam

The MAC address is a unique identifier assigned to network interfaces for communications on the physical network segment. Each network interface card (NIC) comes with a MAC address that is hardcoded into it. This address is used in the data link layer of the OSI model and is essential for the local transmission of data packets.

When devices communicate within the same local network, they utilize MAC addresses to ensure that data is sent to the correct physical device. Unlike IP addresses, which can be changed or assigned dynamically (for example, through DHCP), a MAC address remains constant for the hardware of the device. This permanence is vital for various network functions, including addressing and filtering traffic within the network.

The other options refer to important networking elements but do not uniquely identify a physical interface on a local network in the same way that a MAC address does. While an IP address can identify devices on a network (particularly in a routed environment), it can change often, particularly in dynamic networks. The subnet mask defines the network's size and segment characteristics but does not identify a device itself. A router address serves to identify the router on the network but is not associated with a physical network interface directly. Therefore, the MAC address is the definitive choice for uniquely identifying physical network interfaces.