an_ipv6_function(address) {
if (address & 0xffffffffffff0000 == 0) {
// Use legacy ipv4 protocol ...
} else {
// Use modern ipv6 protocol ...
}
}
a) ping6 ::ffff:127.0.0.1 doesn't work too;
b) it's not an IPv4 address, because the binary representation of ::ffff:127.0.0.1 != 127.0.0.1. It's the IPv4 address space mapped to an IPv6 address space (IPv4-mapped-on-IPv6).
c) IPv4 addresses in IPv6 address space are deprecated: https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.... .
In short, IPv6 and IPv4 cannot coexist, so they must be separated, so we must keep IPv4, because it's mandatory, and we may skip IPv6, because it's optional. So, all internet providers in my area are skipping IPv6, because they can.
All that problems are created by lack of backward compatibility in IPv6 protocol.
Why? «Because backward compatibility with IPv4 is unnecessary. IPv6 is so much better. Everyone will jump to IPv6 till end of the next (1995) year.»
As it is, they locked the upgrade cycle, because you can't do IPv6 only without losing access to IPv4 (historically).
In the end folks did get work arounds going using 464XLAT
Maybe you think it should just be exactly equivalent to ping6 ::1 - your ping6 could be updated to allow this but, like, why? That's not an improvement at all.
OK, so maybe it should be equivalent to ::127.0.0.1 - but that's very strange, an operating system could choose to have this do what you expect locally but it's not clear to what end and again it doesn't offer any wider improvement.
IPv6 is big enough that it contains the entire IPv4 address space - in several different places in fact although ::a.b.c.d is the most transparent. But, that's completely irrelevant to the problem, which I suspect means you still haven't quite grasped what the problem even is.