The IETF/IANA folks were a bit cautious, so only 2000::/3 is officially designated to be assigned. If, a few years down the road, they find out that a mistake was someone made in address allocations, they can then 'start over' from the lessons learned, and start assigning things from 4000::/3.
If they made a mistake again, they can start over again with 6000::/3. And then 8000::/3, a000::/3, and c000::/3.
So that's why all global unicast addresses start with 2: they're to be 'conservative' in what is usable so that if there are problems there is room for corrections in the coming years/decades/centuries.
We don't want to have to go through another IP protocol transition.
Currently not needed.
The IETF/IANA folks were a bit cautious, so only 2000::/3 is officially designated to be assigned. If, a few years down the road, they find out that a mistake was someone made in address allocations, they can then 'start over' from the lessons learned, and start assigning things from 4000::/3.
If they made a mistake again, they can start over again with 6000::/3. And then 8000::/3, a000::/3, and c000::/3.
* https://www.iana.org/assignments/ipv6-address-space/ipv6-add...
So that's why all global unicast addresses start with 2: they're to be 'conservative' in what is usable so that if there are problems there is room for corrections in the coming years/decades/centuries.
We don't want to have to go through another IP protocol transition.