This post is going to be about a topic I have breezed over in the past when reading about MAC addresses but decided to do a little more research on. There are two bits in the first octet of a MAC address that are used to define certain aspects of the MAC address. The least significant bit of the first octet of the MAC address is the Individual/Group(I/G) address bit and the next to least significant bit of the first octet of the MAC address is the Universally or Locally(U/L) administered address bit.
The Individual/Group address bit is used to signify if the destination MAC address is a unicast or a multicast/broadcast Layer 2 address. If the bit is set to 0 then it is an Individual MAC address and is a unicast address. If the bit is set to 1 then it is a Group address and is a multicast/broadcast address. I have not verified this but the logic is reasonable that if a MAC address is unicast then the first octet will never be an odd number when in decimal format.
The Universally Administered or Locally Administered address bit is used to tell if the MAC address is the burned-in-address(BIA) or a MAC address that has been changed locally. Maybe this bit served a purpose in the past but with modern applications on Windows or *nix systems you can change the MAC address to almost anything and this bit does not have to be set to tell the system that you are no longer using the manufacturer’s BIA. If the bit is set to 0 then the MAC address is recognized as a BIA MAC address. When the bit is set to 1 then the MAC address is recognized as being changed from the BIA to a unique MAC address that is locally setup.
Unicast and BIA
Multicast and BIA
Broadcast and Locally Administered