> As you mention in your other post it really is the "billion dollar mistake", it's obvious to me in hindsight that they should have made IPv6 completely backwards compatible with IPv4, no matter the cost.
It's not a matter of cost, it's simply impossible.
IPv4 has a 32 bit field for the source address and a 32 bit field for the destination address, and every connection over IP needs to send packets back and forth between the two addresses. As soon as one party has a longer address, the other party has to know how to handle that, otherwise, they cannot possibly communicate.
The only point where compatibility would be possible to some degree is the network, but (a) that compatibility is in effect a tunnel, and there are lots of ways to tunnel IPv6 over IPv4 just fine, and (b) most of the global internet supports IPv6 just fine, and has been for a long time. The problem is the migration of the endpoints, not so much the network.
It's not a matter of cost, it's simply impossible.
IPv4 has a 32 bit field for the source address and a 32 bit field for the destination address, and every connection over IP needs to send packets back and forth between the two addresses. As soon as one party has a longer address, the other party has to know how to handle that, otherwise, they cannot possibly communicate.
The only point where compatibility would be possible to some degree is the network, but (a) that compatibility is in effect a tunnel, and there are lots of ways to tunnel IPv6 over IPv4 just fine, and (b) most of the global internet supports IPv6 just fine, and has been for a long time. The problem is the migration of the endpoints, not so much the network.