MAC addresses

A MAC address (Media Access Control address) is a unique identifier built into a device's network hardware, such as the network interface card that lets a computer, phone or printer join a network.

Fixed by the manufacturer

The manufacturer writes the MAC address into the hardware when the device is made, and it stays the same for as long as the device exists. It does not change when you join a different network or move the device somewhere else. Every address is meant to be unique: if two devices on the same network shared one, a frame might be sent to the wrong machine and communication would break down.

48 bits, written in hexadecimal

A MAC address is 48 bits long. A string of 48 ones and zeros is slow to read and easy to mistype, so we write it in hexadecimal instead. Each hexadecimal digit stands for 4 bits, so 48 ÷ 4 gives 12 hexadecimal digits. They are written as six two-digit groups; a colon or hyphen normally goes between neighbouring groups, as in 5A:2F:9C:41:B7:D3. Hexadecimal does not change the number of bits stored, but it is much shorter and easier to read and write than binary, which reduces mistakes.

The address is really two halves. The first six hexadecimal digits identify the manufacturer (5A:2F:9C in the example above), and the second six identify that individual device (41:B7:D3).

Used within a local network

MAC addresses do their work inside a single local network (LAN). When a frame arrives at a switch, the switch looks at the MAC address marked as the destination and forwards the frame only through the port that leads to the matching device. Getting data from one network to another is the job of IP addresses.