Is every MAC address unique?

No. A universally administered address is unique inside the block it was allocated from, because the IEEE gives that block to one organization and the block's owner is responsible for handing out the addresses in it. Outside that block nothing enforces uniqueness: an address whose local bit is set is administered locally, and any device or program may choose it.

Where uniqueness comes from

The IEEE Registration Authority assigns address blocks — 24, 28 or 36 bits — to one organization each, and no two organizations hold the same block. An address built from one of those blocks is therefore unique provided two things hold: the organization assigns each address once, and no other device claims it. What an OUI is covers how those blocks are issued.

What breaks it

  • Local administration. Setting the U/L bit — the second hexadecimal digit of the first octet is 2, 6, A or E — declares that the address was assigned by software rather than by a registry. Nothing checks that it is unused.
  • Randomization. A device that generates a fresh address per network fixes two of the 48 bits, the group bit to zero and the local bit to one, and draws the other 46. The space is large, but it is not supervised: two devices can draw the same address, and a device can reuse one it held before. See what a randomized MAC address is.
  • Virtualization and containers. A hypervisor or container runtime invents addresses for interfaces it creates. Docker's default bridge network builds them from 02:42, a prefix no standard reserves, so two hosts running Docker can present the same address to the same network. The Docker interface guide covers that case.
  • Cloning and manual configuration. An address can be copied to a second interface, either deliberately — to keep a licence or a DHCP reservation working — or by accident when a virtual machine is duplicated without regenerating it.

What that means for a lookup

A lookup matches the address against registration data and returns the organization the block belongs to. It answers "which registrant does this prefix belong to", not "is this address unique" and not "which device used it". Two devices presenting the same address return the same answer, and a locally administered address returns no registrant at all because it belongs to no block — why a lookup returns no vendor explains that result. For the wider question of what a match does and does not establish, see what a lookup can identify.

Look one up

Paste an address into the MAC address lookup to see the block it falls in and the registrant that holds it.

Related questions

Sources and review

Last reviewed: . Technical claims on this page follow the primary sources below; operating-system interfaces change between releases, so check them against your own device version.