What is a MAC address?

A MAC address is the 48-bit identifier a network interface uses on a local network. It is written as six hexadecimal pairs, for example 00:1A:2B:3C:4D:5E. The first three pairs name the address block the IEEE Registration Authority assigned to an organization, and the remaining pairs are assigned by the holder of that block, which is why a MAC address lookup resolves an address to a registered organization rather than to a device.

What do the two halves of a MAC address mean?

An EUI-48 — the technical name for the 48-bit form — is an assigned base number plus an extension the assignee controls. In an MA-L block the base is the 24-bit OUI, which is the first three octets, and the block holder fills the last 24 bits. A lookup therefore reports the block, not the individual interface: EC:0D:51 is an MA-L block registered to Apple, Inc. that covers EC:0D:51:00:00:00 through EC:0D:51:FF:FF:FF, and any one of those 16,777,216 addresses resolves to the same registrant. See the EC:0D:51 block for the full record.

Smaller blocks exist and change where the split falls: an MA-M block is identified by a 28-bit prefix and an MA-S block by a 36-bit one, so a lookup has to test more than the first three octets. MA-L, MA-M and MA-S explains the three sizes.

Which bits are not part of the number?

The two least significant bits of the first octet are reserved. The least significant bit is the I/G bit: it is 0 for an individual (unicast) address and 1 for a group (multicast) address. The second least significant bit is the U/L bit: 0 marks a universally administered address, one built from an IEEE assignment, and 1 marks a locally administered address that software or a network administrator chose. Because the U/L bit sits in the first octet, a unicast locally administered address always has 2, 6, A or E as the second hexadecimal digit, as in A6:1B:2C:3D:4E:5F. Addresses of that form are what a randomized MAC address looks like, and they have no registrant to look up.

What else is a MAC address called?

Windows shows it as the physical address, Apple as the Wi-Fi address or hardware address, and Linux tools print it next to HWaddr or link/ether. IEEE and IETF documents call the 48-bit form EUI-48; "MAC-48" is the historical name for the same identifier. IEEE assignments also provide 64-bit EUI-64 values, which other protocols use rather than Ethernet or Wi-Fi addressing.

Does a MAC address identify a device?

Only as far as the registration record goes. An address identifies the organization that registered its block, which is not the same as the model, the owner or the location of the device carrying it. Can a MAC address identify a device? sets out what the record does and does not establish.

Check an address

Paste any address or prefix into the MAC address lookup to see the block it belongs to.