[SoftTalker]

Yes I'm not qualified to really argue the point but naively it never made sense to me that IPv4 was not backwards compatible with IPv6 addressing. You'd think the people on the committees would have foreseen the trouble that would avoid. Telephone companies didn't make you dial the area code for local numbers. Microsoft bent over backwards to make sure that old DOS software still worked on Windows. Linux has a mandate to never break userland. This is not an unfamiliar concept.

[p1mrx]

IP is a bidirectional protocol, so the correct analogy would be if Microsoft had to make DOS software run on Windows, and Windows software run on DOS.

Remember the error "This program cannot be run in DOS mode"?

IPv4 and IPv6 are unidirectionally compatible using NAT64. The fact that you can make an IPv6 to IPv4 TLS connection using a packet-level translator without breaking the endpoints is quite remarkable, and it wouldn't have been possible if the protocols were too dissimilar.