What could have been done better with IPv6
I live in a town in Hungary. The only ISP here is specialized in giving internet connection to small towns, so they don't have service in cities. The connection is IPv4 only. The ISP's address space is not under pressure because due to urbanization most people are going into cities so the amount of addresses needed is actually decreasing for them. So they are not pressured to get more address space. But if they need more address space, they can't get it easily, because all of them are issued already. So they can only get IPv6.
27 years after IPv6 is introduced the transition is still not complete. IPv4 addresses have run out, which means most devices are not actually on the internet instead they talk to a transparent proxy (or NAT) that does the internet connection and talk for them. The problem is that such proxies cannot receive incoming connections and properly forward it to devices unless port forwarding is configured. This means we cannot have proper peer to peer connections anymore.
IPv6 is intended to solve this problem by having a huge address space. But the mortal sin it's makers did that they attempted to build a separate IPv6 internet that has nothing to do with the IPv4 internet. The two is not compatible, and they hoped everyone would pick IPv6 up, so run their system dual stack, then they phase out IPv4.
Needless to say this plan didn't work well, because internet service providers don't want to fix that isn't broken, so they don't deploy IPv6. Even today the primary reason they deploy is that they simply don't have usable IPv4 space anymore. So the only way they can put new devices on the internet is deploying IPv6.
I think this shouldn't be this bad. What should have been done instead is deploying the IPv6 on the top of the existing IPv4 internet. Using automatic tunneling, and using the IPv4 address to derive an IPv6 prefix. That's basically what the 6to4 mechanism is.
But as RFC6343 says, 6to4 had many operating problems. Most of them caused by the fact that the native IPv6 internet exist, and the 6to4 address space does not play nice with the rest of the IPv6 internet. If the native IPv6 internet does not exist, only the 6to4 one, where everyone has an address starting with 2002 most of these problems simply wouldn't exist.
With 6to4 anyone, who has a routable IPv4 address, can get a /48 IPv6 prefix which they can use in their own network. This makes it possible for peer to peer to work even if the two computers are behind home routers and v4 is NAT-ted.
But for some reason we cannot have nice things.
