Fundamentals
How MAC addresses are structured
Forty-eight bits, six bytes, and two of those bits mean something you can read off directly. Here's how the whole address is put together.
A MAC address is 48 bits, written as six bytes. Split down the middle, the two halves have different jobs:
| Bytes | Role |
|---|---|
| First 3 (24 bits) | The OUI: a 24-bit organizationally unique identifier, most commonly a full MA-L assignment. An organization holding a narrower MA-M or MA-S block shares its first 3 bytes with other holders, so a lookup has to match the complete registered prefix, not just these 24 bits, to identify the right one. |
| Last 3 (24 bits) | Assigned by the manufacturer itself, however they like, usually sequentially off a counter. This is what actually tells two of the same vendor's devices apart. |
That split gives one vendor up to about 16.7 million (224) addresses per OUI they hold, which is why IEEE also issues smaller blocks (28-bit and 36-bit, called MA-M, MA-S, IAB, and CID) to organizations that don't need a full 16 million: same idea, just a narrower slice reserved out of a shared 24-bit prefix. Look up any address on our lookup tool and we'll tell you exactly which size block it came from.
The two bits that aren't just address space
Inside the very first byte, two specific bits are reserved to describe the address itself rather than identify anything:
| Bit | 0 | 1 |
|---|---|---|
| I/G (individual/group, the least significant bit of the first byte) | Unicast: addresses one specific interface. | Multicast or broadcast: addresses a group of interfaces at once. |
| U/L (universal/local, the second-least significant bit) | Universally administered: IEEE-assigned, globally unique, burned in at the factory. | Locally administered: assigned by software instead of IEEE, no global uniqueness guarantee. Randomized and virtual-machine MAC addresses set this bit. |
You can read both bits straight off the first hex digit of an address without any lookup at all, since they're the low two bits of that digit. IEEE 802c later formalized the locally administered space further into four "SLAP" quadrants (structured local address plan) for different assignment purposes; our lookup tool decodes all of this, transmission type, administration type, and SLAP quadrant, for any address you give it.
A worked example
Take 00:03:93:AB:12:34. 00:03:93 is Apple's OUI (universally administered, IEEE-assigned), and AB:12:34 is whatever Apple's manufacturing line assigned that specific interface. The first byte, 00, is even, so both the I/G and U/L bits are 0: an ordinary unicast, universally administered address, exactly what you'd expect from a device fresh off the factory floor with no randomization involved.
Curious how this changes when privacy features are involved? See how MAC randomization works. Want the full history of why the format looks like this? See a brief history of the MAC address.
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