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.
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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.