Hello Vadim
MAC addresses can either be universally administered addresses (UAA) or locally administered addresses (LAA). A UAA is one that has been uniquely assigned to a device by its manufacturer using the appropriate OUI and a unique remainder of the address. This ensures uniqueness worldwide.
An LAA is one that is manually assigned by an administrator or a protocol. An example of such a MAC address is the virtual MAC used in HSRP.
So how do you know if a MAC is universally or locally administered? By looking at the 7th bit. This bit is actually called the Universal/Local bit or U/L bit. When it is set to “1” the address is an LAA. When it is set to “0” it is a UAA.
Now, when applied to IPv6 and the EUI-64 process, the meaning of the U/L bit is reversed. Specifically, 1 means universal and 0 means local. This is why, when determining the IPv6 address from the MAC address, this bit is flipped. The result is, if you see this bit as a 1 in the IPv6 address, you know that the original MAC address that it was derived from was a UAA. If you see this bit as 0 in the IPv6 address, you know that the original MAC address that it was derived from was an LAA.
All of this is just a convention, a methodology that is simply applied by definition.
Yes, it can be either. But what it is originally tells us if the MAC address was universally or locally administered.
I hope this has been helpful!
Laz