MAC address format and flag bits explained
How the six octets of a MAC address are structured, what the OUI prefix means, and how the locally administered and multicast bits change the first byte.
Six octets, two halves
A MAC address is 48 bits shown as six hexadecimal pairs, for example 8a:3f:1c:d0:47:9b. Traditionally the first three octets are the Organizationally Unique Identifier, or OUI, which the IEEE assigns to a hardware maker. The last three octets are chosen by that maker to make each card unique. A randomly generated address ignores the vendor registry, which is exactly why the locally administered flag exists.
The two flag bits in the first octet
Only the first octet carries control bits. The lowest bit is the individual or group bit: clear for a unicast address bound to one card, set for a multicast address aimed at many. The next bit up is the universal or local bit: clear for a globally unique factory address, set for a locally administered one. This tool forces those two bits to match your checkboxes and leaves the remaining 46 bits random.
Why random addresses set locally administered
If you generated a fully random address and left the local bit clear, you might accidentally reproduce a real vendor OUI and clash with shipping hardware on a network. Setting the locally administered bit signals that the address was made up on purpose and lives outside the IEEE registry. That is the same mechanism phones and laptops use when they rotate a private MAC address to reduce Wi-Fi tracking.
Reading and comparing addresses
Case and separator never change the meaning: 8A-3F-1C-D0-47-9B and 8a:3f:1c:d0:47:9b are the same address. When you compare values, normalise them to one case and one separator first. If you need to validate that a string is a well-formed MAC, check for six hex pairs joined by a single consistent colon or hyphen, which is the pattern this tool always produces.