2026-07-04 02:13:35

What Is a MAC Address? Structure, Rules, and How It Works

MAC address OUI data link layer ARP networking basics

What Is a MAC Address?

A MAC address (Media Access Control Address) is a unique hardware identifier assigned to a network interface at the data link layer. It is used to identify devices within a local area network (LAN).

A standard MAC address is 48 bits long and written as 12 hexadecimal digits:

00:1A:2B:3C:4D:5E
00-1A-2B-3C-4D-5E
001A.2B3C.4D5E

All three notations represent the same address.

Structure of a MAC Address

A MAC address consists of six bytes (48 bits) split into two halves:

First 3 Bytes: OUI (Organizationally Unique Identifier)

The upper 24 bits identify the manufacturer. The IEEE assigns a unique OUI prefix to each manufacturer — for example, Apple, Intel, Cisco, or Huawei.

This site looks up the first three bytes of a MAC address against the IEEE registry to identify the probable manufacturer.

Last 3 Bytes: Device Serial Number

The lower 24 bits are assigned by the manufacturer to uniquely identify each device it produces.

Rules of MAC Addresses

Fixed Length

Every standard MAC address is exactly 48 bits (6 bytes), written as 12 hexadecimal characters.

Globally Unique vs. Locally Administered

A single bit distinguishes:

Globally Unique (UAA): Assigned by the IEEE and manufacturer, burned into hardware at the factory
Locally Administered (LAA): Configured by software — used by virtual machines or OS-level MAC randomization

Unicast, Multicast, and Broadcast

Unicast: Addressed to a specific device
Multicast: Addressed to a group of devices
Broadcast: FF:FF:FF:FF:FF:FF — sent to all devices on the segment

What Are MAC Addresses Used For?

LAN Communication

Switches forward frames based on MAC addresses, building a CAM table that maps each MAC to a physical port.

ARP Resolution

In IPv4 networks, the Address Resolution Protocol (ARP) translates IP addresses into MAC addresses.

Network Access Control

Routers, Wi-Fi access points, and enterprise equipment use MAC addresses for access policies. Note: MAC addresses can be spoofed and should not be the sole authentication mechanism.

Can a MAC Address Be Changed?

Yes. Although NICs ship with a factory-burned MAC address, it can be overwritten in software by the operating system, a hypervisor, or network tools. Modern smartphones and laptops use randomized MAC addresses when connecting to Wi-Fi to prevent tracking.