Comment by Arainach
5 days ago
It's often impossible to make backwards-compatible changes to a format which wasn't designed to allow for future changes and which is designed to be as space-efficient as possible.
That doesn't mean that the limits of the old design won't hit anyway and force a switch off it.
IPv4 allows future changes. There are some reserved bits in the header that could change a big part of it.
v4 supports extension headers and over a thousand bytes of arbitrary payload so if the only thing you needed was a couple of bits in the packet, there was never any issue with finding them.
The problem is that you can't use those bits to expand v4's address space, without taking all of the same steps v6 needed to do. v4 has no mechanism to get v4 hosts to understand extra address bits, wherever you put them.
Oh, that and the fact that IP addresses are stored in many more places than just the v4 packet header. Consider DNS, DHCP, ARP, gethostbyname(), struct sockaddr_in, databases using VARCHAR(15), etc etc etc. The packet header is only a tiny part of the story.