Media Access Control (MAC) address
A hardware identifier, 48 bits in its common form, assigned to a network interface and used to deliver frames on a local network segment at the Data Link layer.
A MAC address identifies a network interface on a local network. In the common EUI-48 format administered by the IEEE, it is 48 bits long and written as six pairs of hexadecimal digits, such as 00-00-5E-00-53-01. In a globally unique address, the first half is an organisationally unique identifier that the IEEE assigns to the manufacturer, which numbers the rest. The address works at layer 2, the Data Link layer, of the OSI model: switches forward frames by it, and it does not travel beyond the local segment, because routers replace the layer 2 header at each hop. On IPv4 networks, ARP maps a logical IP address to the MAC address needed for delivery.
A MAC address is often described as burned in, yet software can change the address an interface presents easily. That makes it weak as an identity: MAC filtering on a Wi-Fi network or a switch port can be bypassed by copying an allowed address, and forged addresses underpin MAC flooding and ARP spoofing.
Exam relevance: questions in this area tend to turn on the layer a MAC address belongs to and how it differs from an IP address. A scenario is likely to reward the candidate who treats MAC-based controls as easily bypassed rather than as authentication.