A long time ago, in another country, JANET had a mail list to discuss email, in a world before DNS. And, when DNS emerged, JANET mail list made a *deliberate* decision to make the domain order of UK email domains the reverse of every other country worldwide. A DELIBERATE decision. (I was there, on this list. Others may disagree with my interpretation of acts done and motivations, but I want to be clear I didn't "hear this second hand" -I was receiving the mailflows discussing this in public. I am sure there are other private conversations I didnt see) It wasn't a consensus decision. It wasn't an entirely rational decision. OTOH it was a research network, email was a research activity, and in some ways, it made sense to find out what happened. That the decision had repercussions which echoed down the years, and marginalized some communications Uk and internationally, is perhaps, the real lesson. IPv6 had an opportunity to consider designs which were intermediate, (IPv4-and-a-bit) and backwards compatible. And, like JANET and domain order, people decided not to do it, believing it was interesting and research-y. I too wish we had selected TUBA, or had thought more about interop with IPv4. I sometimes wish I understood why SRC was the first element off the wire, and not DST, Since rational ASIC/FPGA hardware can latch early on the SRC and begin routing faster if it appears in natural bit order first. Or, why we even have SRC in the header: it does not inform routing. These are heresies. Counterfactuals dog Historians. Some love them, some hate them. We don't have time machines. This is the world we live in, we have to make the best of it we can. IPv6 globally is rising, IPv6 in Asia is rising. IPv6 in India is basically ubiquitous, IPv6 in America is ubiquitous. We are going to live in a mixed protocol global internet for the forseeable future. We can plan to extend V4, or end V4, or deprecate V4, or end v6, and favour CGN but we can't end either V4 or V6 entirely, easily, soon. -G