MAC Address

Media Access Control address: a 48-bit hexadecimal number used to identify computer systems by their NICs.

The MAC address is hardcoded into the NIC's readonly memory during manufacturing.

Properties

All MAC addresses are unique. The first half of the 48-bit identifier is used to pinpoint the manufacturer. Every NIC manufacturer in the world is assigned a unique six-digit sequence by the IEEE. The rest of the number is randomized, unique to that device.

Any data packet sent by a computer has it's NIC's MAC address embedded somewhere in the packet's frame data.