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Comment by bradley13 | Hacker News Reader

No, it's not. If I have an ipv6 network, an ipv4 address is invalid. It's that simple.

It's not that simple at all. For one thing, having a v6 network doesn't mean you can't have a v4 network. You can run v4 in exactly the same way you currently do, with exactly the same software, and it'll work no worse than it already does.

But for another, the v4 space is available as a subset of the v6 space:

  $ ping 64:ff9b::8.8.8.8
  PING 64:ff9b::8.8.8.8(64:ff9b::808:808) 56 data bytes
  64 bytes from 64:ff9b::808:808: icmp_seq=1 ttl=113 time=9.82 ms

That's from a machine on a network with no v4, and it works fine. I can reach v4-only sites from it too. I could even do this using v4 addresses if I wanted, but if I showed you the output from that you'd just claim I was using v4.

The point of backwards compatibility would be to allow IPv4 devices to work on an IPv6 network. Not to run a parallel stack.

127.0.0.1 needed to be a valid IPv6 address, along with all the others. Pick a particular prefix, say 0...* and any address with that would be extended to 128 bits. That would have been backwards compatible.

No, that would be forwards compatibility. v4 doesn't have forwards compatibility with any address protocol that uses addresses bigger than 32 bits, and it never will regardless of how that protocol is designed because the flaw is in v4's design.

There is no possible way to design an address protocol with bigger addresses than v4 that a) makes v4 forward compatible with it, and b) can actually work. Feel free to suggest one.

> .0.0.1 needed to be a valid IPv6 address, along with all the others. Pick a particular prefix, say 0...* and any address with that would be extended to 128 bits

That prefix is ::ffff:0:0/96. 127.0.0.1 is ::ffff:127.0.0.1 (::ffff:7f00:1). 30 years and you still haven't realized v6 has this?

No, it’s not—IPv6 networks are totally capable of providing IPv4 as a service. SIIT-DC, 464XLAT, MAP-T

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