All the proposed variants required that. Some hid the requirement a little better than others.
Some proposals were superset-like, so v4 and v6 addresses could ping each other. But not all v6 addresses. As soon as the v4 space was used up, those variants had to allocate v6 addresses that could not ping v4 addresses, so you got a sneaking incompatibility. Worse: you'd never know for sure whether there were any v4-only hosts left on the network.
Clean break with the new protocol NATing the old protocol. IPv4 addresses not behind the NAT are not accessible on the new internet. Exploit the asymmetry inherit in services vs clients.
Comments
All the proposed variants required that. Some hid the requirement a little better than others.
Some proposals were superset-like, so v4 and v6 addresses could ping each other. But not all v6 addresses. As soon as the v4 space was used up, those variants had to allocate v6 addresses that could not ping v4 addresses, so you got a sneaking incompatibility. Worse: you'd never know for sure whether there were any v4-only hosts left on the network.
Clean break or sneaking, what's your preference?
Clean break with the new protocol NATing the old protocol. IPv4 addresses not behind the NAT are not accessible on the new internet. Exploit the asymmetry inherit in services vs clients.