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Comment by eqvinox | Hacker News Reader

> What I don’t understand is why coexistence was so important.

Military, corporate, tech... it isn't. (If your people like flag day migrations. It's… "a choice".) But if you have to explain to an end user why some things work and some don't, you're just f'd.

And note "coexistence" here means that an end host can implement IPv4 and IPv6 at the same time, without them interacting at all. Imagine if you had to choose between IPv4 and IPv6 on your devices, maybe something like "you need a 2nd network card". Can you imagine anyone switching to the less popular protocol?

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The article describes coexistence as both dual-stack and connectivity between single-stack IPv6 and single-stack IPv4 host. And that in the autor's opinion all the complexity is in the latter, not in the dual-stack

You raise a good point that we also should't take dual- stack for granted. But I think the more precise question 'why not dual-stack as the only coexistence option' also seems like a good one, and one the article does not explore or even acknowledge

Dual-stack was the only coexistence option for a long time, until NAT64 came around. There were a whole bunch of attempts at compatibility, e.g. with "::1.1.1.1" and "::ffff:1.1.1.1" as IPv6 addresses, they just didn't go anywhere. (Well, not quite, the latter is in POSIX and in socket libraries around the planet. Doesn't leave the host though. At least it's not supposed to. I have some horror stories…)

NAT64 started happening because it solves real problems — large eyeball networks, particularly mobile phone networks, didn't want to pay for twice as large table sizes on their routers and twice the maintenance effort. So they made IPv6 end hosts capable of connecting to IPv4 systems. But this is 2010 era, IPv6 was ≈15 years old at that point!

NAT64 is a subset of a different thing that existed since 2000 though, when v6 was ~5 years old and before most OSs even had support for it.

What would that "thing" be?

See RFC 2766.

Ah, right, that one with all the ALGs. Kinda NAT64's ancestor, though I have no idea of the evolution/development process.

I don't believe this ever worked at scale, even if it is pretty much NAT64. Particularly the part where IPv4 hosts can reach IPv6 systems with the NAT-PT tracking through DNS what is given out in A records is "a bit much".

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