What makes a MAC address valid?

A MAC address is 48 bits, written as six octets in hexadecimal. Any six-octet hex value is technically valid as an address. The two lowest bits of the first octet carry meaning: bit 0 is the unicast or multicast flag and bit 1 is the universal or locally-administered flag.

What is the locally-administered bit?

Setting the second-least-significant bit of the first octet marks an address as locally administered, meaning it is not a globally unique vendor-assigned address. Random test MAC addresses should usually set this bit so they cannot collide with real OUI-assigned hardware.

What is the difference between unicast and multicast?

If the least-significant bit of the first octet is 0 the address is unicast, meant for a single interface. If it is 1 the address is multicast, addressed to a group. Most device addresses are unicast; this tool defaults to unicast but lets you generate multicast addresses too.

Which separator should I use?

Colon-separated lower case (aa:bb:cc:dd:ee:ff) is common on Linux and macOS, hyphen-separated upper case (AA-BB-CC-DD-EE-FF) is common on Windows, and the dotted triplet form (aabb.ccdd.eeff) is used by Cisco devices. Pick whatever your target system expects.

Are these real registered addresses?

No. The octets are random, so the OUI portion will not generally match a registered vendor. Enable the locally-administered bit to guarantee the address is in the private space and will never clash with real hardware.

MAC Address Generator

A MAC address is the 48-bit hardware identifier carried by every network interface, written as six hexadecimal octets. This generator produces correctly formatted random MAC addresses with the separator, case, and control-bit options you need for network testing, mock device inventories, and configuration scripts.

How it works

Six random octets are generated, then the first octet is adjusted to set the two control bits correctly:

first octet bit 0 (0x01) = I/G bit : 0 = unicast, 1 = multicast
first octet bit 1 (0x02) = U/L bit : 0 = universal (OUI), 1 = locally administered

When you ask for a locally-administered address the tool ORs 0x02 into the first octet so the U/L bit is set; for a universal address it clears that bit. Likewise the I/G bit is set or cleared for multicast or unicast. The six octets are then joined using your chosen separator and case.

Tips and notes

For throwaway test addresses, keep the locally-administered bit on so they live in the private range and can never collide with a real vendor-assigned address. Match the separator to your platform — colons for Linux and macOS, hyphens for Windows, dotted triplets for Cisco IOS. If you need a deterministic set for repeatable tests, generate once and store the list rather than regenerating, since each batch is random.