MAC Address or Ethernet Addressing Explained

Every Ethernet frame contains two addresses: source and destination. The source address represents the device that generated it. The destination address represents the recipients of the frame.

An Ethernet address is also known as a Hardware address, physical address, burned-in address, universal address, MAC address, or LAN address.

These terms define the purposes and functions of the Ethernet address. For example, the terms hardware address and physical address indicate the address belongs to an interface.

The terms MAC address and LAN address indicate the data link layer uses this address in the LAN environment.

The term burned-in address (BIA) specifies a fact that indicates a permanent MAC address has been encoded (burned into) the ROM chip on the NIC.

The term universal address indicates the address is unique in the universe.

Globally unique MAC addresses

An administrative process is followed to make each MAC address unique in the universe. A MAC address is a 6-byte-long (48-bit-long) binary number. In this number, the first 3 bytes are assigned by IEEE and the last 3 bytes are assigned by the manufacturer.

Before a manufacturer builds Ethernets, it obtains a universally unique 3 bytes code from IEEE. IEEE provides a unique 3 bytes code to every Ethernet product manufacturer.

The manufacturer uses the assigned code to generate MAC addresses for its products. In each MAC address, it uses the assigned code as the first 3 bytes. It uses the last 3 bytes to make the address unique. As a result, the MAC address of every device in the universe is unique.

The following image shows an example of this process.

globally unique MAC address

For convenience, devices display MAC addresses as 12-digit hexadecimal numbers. They add periods between two hexa numbers. For example, a Cisco switch might list a MAC address as 0012.AB12.3456.

Examples of MAC addresses

Following are the example MAC addresses

0000.AB12.3456, AA12.AB12.3456, 0012:1234:45CD, CC00:AABB:CC22

Key points

  • MAC stands for Media Access Control.
  • Each MAC address is unique in the universe.
  • MAC addresses work in the Data Link layer.
  • A MAC address is locally significant.
  • A MAC address is 48 bits long in binary.
  • A MAC address is usually written in hexadecimal.
  • An Organizationally Unique Identifier is a 3-hexa bytes code.
  • IEEE assigns OUI codes to Ethernet manufacturers.
  • OUI codes are unique among manufacturers.
  • Manufacturers use OUI codes to generate unique MAC addresses for their products.
  • In each MAC address, the first 3-hexa bytes are OUI.
  • The manufacturer uses the last 3-hexa bytes to generate unique MAC addresses for every interface.

ComputerNetworkingNotes CCNA Study Guide MAC Address or Ethernet Addressing Explained