While troubleshooting a wireless issue, I mentioned offhand to another engineer that a particular MAC address was private. They immediately asked me “How did you know just by looking at it?”
I said “Look at the second least significant bit of the most significant byte,” but quickly realized that needed a bit more explanation.
The Local bit is zero for globally unique EUI-48 identifiers assigned by the owner of an OUI or owner of a longer prefix. If the Local bit is a one, the identifier has been considered by IEEE 802 to be a local identifier under the control of the local network administrator […]
This is the
02 bit of the first octet in the MAC. If it is set, this
is a locally-administered address. Essentially, if the second hex digit
E, it is a private MAC.
What about MAC addresses with the next bit set, such as when the first
07? Those still have the local bit set. Yes,
01 bit is the unicast/multicast bit (individual/group, I/G
bit). We seldom see use of locally-administered multicast
layer 2 addresses, so can be ignored for the purposes of “private MAC