Unique Local Addresses

Unique local IPv6 addresses are private addresses that cannot be used over the internet.

You do not need to register to use them. They can be used freely within internal networks and don't need to be globally unique. Can't be routed over the internet.

The global ID should be unique so that addresses don't overlap when companies merge.

Uses the address block FC00::/7 (FC00:: to FDFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF).

However, a later update requires the 8th bit to be set to 1, so the first two digits must be FD.