There is as good interoperability as is possible with various transition technologies allowing IPv4 and IPv6 hosts to reach other, with various variants of NATs or tunneling of IPv4 over IPv6 and vice-versa, mapping individual IPv6 addresses to IPv4, mapping the IPv4 space into IPv6, etc. etc.
The basic problem is though that IPv4 has no forward compatibility.
There is no way to build anything that has a larger address space than IPv4 and remain reachable for an IPv4-only host, since the IPv4 header has fixed 32-bit address fields.
This alone is the main limiting factor of the transition (combined with people refusing to dual-stack).
The basic problem is though that IPv4 has no forward compatibility. There is no way to build anything that has a larger address space than IPv4 and remain reachable for an IPv4-only host, since the IPv4 header has fixed 32-bit address fields. This alone is the main limiting factor of the transition (combined with people refusing to dual-stack).