[lisper]

I was just writing up my own comment to say exactly this when I noticed your comment. It's particularly unfortunate that ipv6 was not made backwards-compatible because it would not have been difficult to do: extra address bits could have been included in the options portion of the header. An IPv4 packet could be routed over an IPv6 network simply by setting the (missing) extended address bits to some canonical default value (most likely 0). The net result would be similar to an HTTP/1.0 client accessing an HTTP/1.1 server with multiple virtual servers running on it: accessing an IPv4 host at A.B.C.D would be the same as accessing an IPv6 host at A.B.C.D.0....0

Such a missed opportunity.