It would have been so easy if the early Internet and TCP/IP network designers had made IPv6 backward compatible with IPv4. They didn’t. In 1981, IPv4’s 32-bit 4.3 billion addresses look more than enough addresses for the ARPANet/Internet. That was the Internet then, this is the Internet now.
Oh, network professionals saw the Internet address shortage coming and knew it would be a problem. I can’t do better than to quote, Leslie Daigle, Chief Internet Technology Officer for the Internet Society, who admitted at a June 2009 meeting that “IPv6’s lack of real backwards compatibility for IPv4 was [its] single critical failure.” It’s too late now to cry over spilled standards. We need to work on getting the two fundamental network standards to peacefully co-operate today.