Switching - Media Access Control (MAC) addresses

A Media Access Control (MAC) address is a unique identifier that is assigned to a network interface controller such as an Ethernet card, or a switch port, that is used for communication within a network segment. It is an unmodifiable hardware address, sometimes called a burned-in address, or physical address.

Network devices that operate on Layer 3 such as routers as well as multi-layer switches will have multiple MAC addresses, one assigned to each network interface. MACs must be unique on a network segment, but may not be on different network segments.

A MAC address is 48 bits or 6 bytes in total.  We write it in hexadecimal. For example:


Above we have four hexadecimal characters, separated by a dot. There are two other formatting options as well:

00:00:0c:12:34:56 00-00-0c-12-34-56

All three refer to the same MAC address. Cisco devices often use the first formatting option, Windows computers use the dash in between.

Normally, a MAC address refers to a single device on the network. We call this a unicast MAC address. There is also a MAC address for broadcast traffic (which means that everyone on the network will receive the frame) or multicast traffic (a group of receivers).

MAC addresses have to be unique, otherwise, it’s possible that an Ethernet frame ends up at two receivers. To make MAC addresses unique, each networking vendor that wants to build network cards has to ask the IEEE for a unique 24-bit code called the OUI (Organizationally Unique Identifier).

For example, all MAC addresses that start with 0000.0c are owned by Cisco.

The remaining 24 bits of the MAC address are then assigned by the vendor. Each network card that they create will have a unique MAC address. The address that the vendor has assigned is also called the BIA (burned-in address).