How do we identify an address as a valid MAC address

Question

If 000.a83.b1c.08e is a valid MAC address then why is 123.123.123.123 not a valid address? Also if how do we identify its 32 bit long or 48 bit long

Answer 1

The standard notation for 48-bit MAC addresses is with 12 hexadecimal digits (0-9,a-f), often grouped in two, four, or six, rarely three.

32-bit IPv4 addresses are usually written as four decimal octets, 0-255 each, separated by dots.

123.123.123.123 looks very much like an IPv4 address, but could actually be interpreted as a MAC address as well (hexadecimal, in groups of three), 12-31-23-12-31-23 in IEEE notation. However, 12... indicates a locally-administered address (the U/L bit is set) which might not be accepted in many environments.

Similarly, any odd value in the second digit would indicate a group address (I/G bit set) which cannot be used as a source address in general. 0, 4, 8, or c in the second hex digit would indicate a valid, global, unicast address.