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Dan J. Bernstein wrote about the reasons why, and it's quite enlightening. https://cr.yp.to/djbdns/ipv6mess.html https://cr.yp.to/djbdns/ipv6mess.html

lmm 3y ago
No it isn't. That page sounds clever to the clueless but to anyone who actually understands that packet-switched networks won out over circuit-switched ones it's deeply stupid, to the point that I wonder if he wrote it as some kind of deep cover trolling.
- NoZebra120vClip 3y ago
  I suppose that you could argue that packet switching won out over circuit switching. I mean, except for the gigantic installed base of landline telephones (analog and ISDN) and the digital switches that support them on copper pairs, still extant and supported, VoIP is incrementally replacing those lines as the decades progress. (How many decades has it been now?) But you're comparing apples and oranges. What was the size of the established, installed base of network hosts using circuit-switched networks when packet switching started to compete? Did SRI International and other DARPA research institutions run IMPs that utilized circuit-switching to route network traffic? No, they started with packet switching. So essentially, as the Internet caught on, its style of packet switching "won" over landline telephones because it was a question of capability and supported features. IPv4 and IPv6 are the same protocol, essentially; you swap one out under the hood, and the upper and lower OSI layers don't even notice that anything's changed. Packet switching and circuit switching are two distinct techniques that are opposed to one another: if you wanted to swap one for the other, you'd be rebuilding your network from the ground up. So, apples and oranges: IPv6 is, by design, a replacement for IPv4, and djb is discussing the seemingly intractable issues faced by those who attempt to cleanly migrate.
- ianburrell 3y ago
  Also, the article is old. It doesn't mention NAT64 which got rid of the big compatibility issue of IPv6 hosts not being able to talk to IPv4 servers. The other way, IPv4 to IPv6, will never happen and could never happen with larger address space. People talk about extending IPv4 address space but never explain how they would solve the compatibility. I thought of scheme where could put the original IPv4 address in option header on every NAT transition to make a variable length address. But that would require changing every router (and lots remove options when NAT), every program, and every protocol. All the stuff that needed to happen with IPv6, with the downside of keeping IPv4 allocations and NAT. It also mentions a lot of problems with IPv6 that have since been fixed. There is a valid complaint that IPv6 changed too many things so it took a long time to get replacements working.

lmm 3y ago

No it isn't. That page sounds clever to the clueless but to anyone who actually understands that packet-switched networks won out over circuit-switched ones it's deeply stupid, to the point that I wonder if he wrote it as some kind of deep cover trolling.

NoZebra120vClip 3y ago
I suppose that you could argue that packet switching won out over circuit switching. I mean, except for the gigantic installed base of landline telephones (analog and ISDN) and the digital switches that support them on copper pairs, still extant and supported, VoIP is incrementally replacing those lines as the decades progress. (How many decades has it been now?) But you're comparing apples and oranges. What was the size of the established, installed base of network hosts using circuit-switched networks when packet switching started to compete? Did SRI International and other DARPA research institutions run IMPs that utilized circuit-switching to route network traffic? No, they started with packet switching. So essentially, as the Internet caught on, its style of packet switching "won" over landline telephones because it was a question of capability and supported features. IPv4 and IPv6 are the same protocol, essentially; you swap one out under the hood, and the upper and lower OSI layers don't even notice that anything's changed. Packet switching and circuit switching are two distinct techniques that are opposed to one another: if you wanted to swap one for the other, you'd be rebuilding your network from the ground up. So, apples and oranges: IPv6 is, by design, a replacement for IPv4, and djb is discussing the seemingly intractable issues faced by those who attempt to cleanly migrate.
ianburrell 3y ago
Also, the article is old. It doesn't mention NAT64 which got rid of the big compatibility issue of IPv6 hosts not being able to talk to IPv4 servers. The other way, IPv4 to IPv6, will never happen and could never happen with larger address space. People talk about extending IPv4 address space but never explain how they would solve the compatibility. I thought of scheme where could put the original IPv4 address in option header on every NAT transition to make a variable length address. But that would require changing every router (and lots remove options when NAT), every program, and every protocol. All the stuff that needed to happen with IPv6, with the downside of keeping IPv4 allocations and NAT. It also mentions a lot of problems with IPv6 that have since been fixed. There is a valid complaint that IPv6 changed too many things so it took a long time to get replacements working.