IPv6 is the poster child for the second system effect (or solution) [1].

IPv4 really only had 3 problems that anybody cared about:

1. Address space size;

2. Roaming; and

3. Reliable connectionless delivery; and

4. The problems created by the at most once delivery under TCP when what we really needed was at least once delivery in many, many cases.

Even the address space size problem is less of an issue than originally predicted because of improvements in NAT, up to and including cgNAT for cellular network providers (which also somewhat addressed (2) in a limited way).

Interestingly, some of the larger companies have networks simply too large for the 10.0.0.0/8 address space.

By "roaming" I mean maintaining a consistent connection while moving between networks.

(4) has kinda fallen on QUIC (now HTTP3) but this should really be core TCP/IP Layer 3.

You could also say that TCP congestion control is pretty outdated. It's not surprising. It was designed at a time before megabit (let alone gigabit) networks. And, more importantly, latency kills throughput. Some efforts have been made on this, such as Google's BBR [2], but other problems remain like MTU windows being too small for modern networks.

So what did IPv6 do? It only solved one problem, address space, and it did it in a way that kinda created new problems. First, the address space is too large (128 bits) and the last 64 bits are kinda reserved for the job that a 16 port used to do. And why was that? Originally, it was intended that the lower 64 bits were derived from a 48 bit MAC address (as used by Ethernet and later Wifi) but they realized this was a huge privacy problem so it never happened.

[1]: https://en.wikipedia.org/wiki/Second-system_effect

[2]: https://github.com/google/bbr

[2]: https://community.cisco.com/t5/networking-knowledge-base/und...