Random MAC Generator
Locally administered, unicast
The second bit of the first octet is set and the first is clear, so these cannot collide with a manufacturer's assigned range.
The random MAC generator produces six-byte addresses in four notations, colon, hyphen, Cisco dotted and bare hex. Every generated address is locally administered and unicast, which means it cannot collide with a manufacturer’s globally assigned range.
How it works
Two bits in the first byte decide what kind of address it is, and setting them correctly is the whole difference between a safe test address and one that could clash with real hardware.
- Bit 0 of the first byte is the individual/group bit. Clear means unicast; set means multicast.
- Bit 1 is the universal/local bit. Set means locally administered, not assigned by the IEEE to any vendor.
- The generator forces bit 1 on and bit 0 off, so the first byte always has 2, 6, A or E as its second hex digit.
- The remaining five bytes are uniform random.
Examples
Colon notation
Notation
aa:bb:cc:dd:ee:ff
How many
3
Result
9a:3f:c1:08:d7:44, 4e:b2:70:1c:e9:05, 6a:5d:33:be:07:91
The second hex digit of the first byte is a, e or a in these three, always one of 2, 6, A or E, which is the visible signature of a locally administered address.
Cisco dotted notation
Notation
aabb.ccdd.eeff
Result
9a3f.c108.d744
Cisco equipment groups MAC addresses into three four-digit blocks rather than six pairs. Same address, different presentation.
Frequently asked questions
What makes a MAC address "locally administered"?
The second-least-significant bit of the first byte. When set, the address is declared as assigned locally rather than by the IEEE, so it cannot conflict with a manufacturer’s allocated block. You can spot one by eye: the second hex digit is always 2, 6, A or E.
Can I use a generated address to change my device’s MAC?
Technically yes on most systems, and using a locally administered address is the correct way to do it. Be aware that some networks tie access to a registered address, so changing it can lock you out.
Do the first three bytes identify a manufacturer?
In a real address, yes. The first three bytes are the Organisationally Unique Identifier assigned to a vendor. These are deliberately not real OUIs, because using a vendor prefix you do not own is what causes collisions on a live network.
Why does notation matter?
Because tools disagree. Linux and macOS print colon-separated lowercase, Windows uses hyphens and uppercase, and Cisco groups four digits at a time. Software that parses only one form is a common source of bugs, which makes all four useful as test input.