MAC Address Format: OUI, Local Addresses and Test Examples

MAC address format for Ethernet and Wi-Fi is six octets, or 48 bits. 00:00:5e:00:53:01 is an example from the IANA documentation block. It labels a network interface on a local link; it is not a person identifier or proof of a device manufacturer ^[rfc7042].

Read the format before looking up an OUI

Pick which input spellings your application accepts, validate the complete format, and then normalize. Blindly deleting punctuation can turn malformed input into an accepted address. Do not strip arbitrary characters and assume the remaining twelve digits prove the original input was valid.

What the first-octet bits mean

The least significant bit of the first octet is I/G: 0 for an individual address and 1 for a group address. The next bit is U/L: 0 for universal administration and 1 for local administration. For a locally administered unicast address, (firstByte & 0x03) === 0x02 ^{[ieee-guidelines]}.

Why an OUI lookup may not identify the hardware

IEEE registries describe assignments to organizations. The familiar first-three-byte lookup covers MA-L assignments, but smaller allocation products use longer prefixes. Lookup software needs the matching registry and longest applicable prefix; it should return unknown for a local address instead of asserting a vendor ^[ieee-ra].

Even a registry match identifies the registered organization, not necessarily the retail brand or the current user. A software-configured address is another reason to treat vendor lookup as descriptive metadata rather than authentication.

Documentation addresses versus working test interfaces

For published examples, use the 256 unicast values from 00:00:5e:00:53:00 through 00:00:5e:00:53:ff. For group-address examples, RFC 7042 also provides 01:00:5e:90:10:00 through 01:00:5e:90:10:ff. Those are specific documentation allocations ^[rfc7042].

For a VM or interface on an isolated test network, locally administered unicast addresses are appropriate when you control allocation. They are also used by real devices: Apple documents private Wi-Fi addresses for its hardware ^{[apple-private]}. A local bit cannot distinguish synthetic traffic from production traffic.

A worked local-address mask

Start with six random bytes. For a first byte of hexadecimal AD, applying (0xad & 0xfc) | 0x02 produces AE. Clearing the bottom two bits and then setting bit 1 leaves a local unicast address. The possible second hexadecimal digits are 2, 6, A and E.

This leaves 46 variable bits, or 70,368,744,177,664 possible local unicast values. That is a large space, not a uniqueness guarantee. Keep an allocation set for a test network and retry on duplicates. Fixed sequential values may be easier to inspect than random ones in a small isolated lab.

MAC parser test cases and expected results

Fakenamely’s current identity and bulk schemas do not export a MAC field. Use these examples or implement the mask in your fixture builder. Pair it with the IP range guide for Layer 3 tests; keep IP routing policy separate from MAC format validation.