The last 64 bits

The last 64-bits are the interface or host ID. Originally, to ensure uniqueness, there was a standard called Extended Universal Identifier or EUI-64 for stateless configuration. This format expands the 48-bit MAC address to 64 bits by inserting “FFFE” into the middle 16 bits between the upper 3 bytes (Organizational Unique Identifier field) and the lower 3 bytes (serial number) of the link layer address. To make sure that the chosen address is from a unique Ethernet MAC address, the universal/local bit (bit 7) is set to 1 for global scope or 0 for local scope.

There are huge privacy issues with this and it is not now generally used, check [9]. Most operating system now generate a random 64-bit address.