What is a MAC address?

A MAC (Media Access Control) address is a unique identifier assigned to a network interface controller (NIC) for communications on a physical network segment. It is also known as a hardware address, Ethernet address, or physical address. The MAC address is used to identify devices on a local network, such as computers, printers, routers, and other networking devices.

Here's a technical breakdown of a MAC address:

  1. Format:
    • A MAC address is a 48-bit (6-byte) unique identifier, typically represented in hexadecimal notation. For example, a MAC address might look like this: 00:1A:2B:3C:4D:5E.
  2. OUI (Organizationally Unique Identifier):
    • The first 24 bits (3 bytes) of a MAC address represent the OUI, which is assigned to the manufacturer or vendor of the network interface card. The OUI is administered by the Institute of Electrical and Electronics Engineers (IEEE).
  3. NIC Identifier:
    • The remaining 24 bits (3 bytes) uniquely identify the specific network interface card within the manufacturer's allocation. This part is assigned by the manufacturer and is meant to be unique for each NIC they produce.
  4. Uniqueness:
    • The combination of the OUI and the NIC identifier ensures that each MAC address is globally unique. However, it's important to note that in practice, the uniqueness of MAC addresses is not always guaranteed, as some manufacturers may reuse MAC addresses.
  5. Broadcast Domain:
    • MAC addresses play a crucial role in the Ethernet protocol for addressing frames within a local network. Each device on a network has a unique MAC address, and when data is sent over the network, it is addressed to a specific MAC address. Broadcast messages, which are meant for all devices on the network, use a special MAC address (all ones or all zeros, depending on the context).
  6. Layer 2 Addressing:
    • MAC addresses operate at the Data Link Layer (Layer 2) of the OSI model. They are used by network devices to make decisions about how to forward data frames within a local network segment.
  7. Locally Administered Addresses:
    • In addition to the OUI-assigned MAC addresses, devices may also use locally administered addresses. These are MAC addresses that are manually assigned by network administrators, and they do not follow the OUI structure.