What Is a MAC Address?
A MAC (Media Access Control) address is a unique 48-bit identifier assigned to a network interface card (NIC) by its manufacturer. It is used at the data link layer (OSI Layer 2) to identify devices on a local network segment. Unlike an IP address, a MAC address is not routable across the internet and never leaves the local network. See the IP vs MAC address comparison for a side-by-side breakdown.
MAC Address Format and Structure
| Component | Bits | Description | Example |
|---|---|---|---|
| Full MAC Address | 48 bits (6 bytes) | Written as 6 hex octets separated by colons or hyphens | 00:1A:2B:3C:4D:5E |
| OUI (Organisationally Unique Identifier) | First 24 bits (3 bytes) | Assigned to manufacturers by IEEE - Identifies the vendor | 00:1A:2B = Intel, 3C:22:FB = Apple |
| NIC-specific portion | Last 24 bits (3 bytes) | Assigned by the manufacturer to uniquely identify the interface | 3C:4D:5E |
| Unicast/Multicast bit | Bit 0 of first byte | 0 = unicast (individual device); 1 = multicast (group) | 00 = unicast |
| Universal/Local bit | Bit 1 of first byte | 0 = globally unique (burned-in by manufacturer); 1 = locally administered (randomised or manually set) | 2 in second nibble = locally administered |
Burned-In vs Locally Administered MAC Addresses
The original MAC address burned into the NIC hardware by the manufacturer is globally unique. However, operating systems can override this with a locally administered address set in software - Commonly called MAC spoofing or MAC randomisation. When a MAC address is locally administered, the second-least-significant bit of the first byte is set to 1, which is how you can identify a randomised address.
MAC Randomisation for Privacy
- iOS 14+, Android 8+, and Windows 10 1903+ all randomise MAC addresses per Wi-Fi network by default to prevent tracking across access points.
- Wi-Fi access points and network analytics systems can track which MAC addresses visit a location and how frequently - MAC randomisation defeats this tracking at the hardware level.
- MAC randomisation generates a new pseudo-random locally administered MAC address each time your device connects to a new (or previously randomised) Wi-Fi network.
- Some enterprise networks use MAC address filtering as an access control - MAC randomisation can cause issues on these networks, requiring you to disable it for that specific network.
- Your MAC address is only visible within your local network - It is not transmitted in IP packets and is never seen by websites you visit. Websites instead see your public IP address.