MAC Address Generator
Sets the locally administered bit and clears the multicast bit, so the addresses cannot collide with a real vendor assignment.
Runs in your browser
Network
First address, bit by bit
The two bits that matter
The first octet of a MAC address carries two flags in its lowest bits:
- Bit 0 — unicast (0) or multicast (1)
- Must be 0 for a host address. Set it and switches treat the frame as multicast, which presents as traffic arriving but never being answered. This generator always clears it, so the first octet is always even.
- Bit 1 — universal (0) or local (1)
-
Set means the address was assigned locally rather than by a manufacturer, and so cannot
collide with any real NIC. This is what makes
02:,06:,0a:and0e:the safe prefixes for anything you invent.
Choosing a hypervisor's registered OUI instead keeps the address recognisable in a MAC table, which is worth more operationally than strict correctness — those ranges are assigned to the vendor and will not clash with your physical estate.
Practical notes
- Duplicates break silently
-
Two hosts with the same MAC on one segment produce intermittent loss, not a clean failure —
the switch's MAC table flaps between ports. Cloning a VM without regenerating its address is
the usual cause. Check
show mac address-tablefor the address appearing on two ports. - DHCP reservations key on this
- Regenerating a MAC invalidates the reservation, and the host quietly picks up a dynamic address instead. Change one or the other, not both at once.
- IPv6 SLAAC may expose it
- Classic EUI-64 addressing embeds the MAC in the IPv6 address, making the host trackable across networks. RFC 8981 privacy extensions are the default on modern systems, but check on servers — they are frequently disabled there.