It's very rare to make a breaking change in a protocol as ubiquitous as IP. One thing about making a breaking change is that once you've committed to one such c

It's very rare to make a breaking change in a protocol as ubiquitous as IP. One thing about making a breaking change is that once you've committed to one such change it becomes much cheaper to bundle in other changes at the same time. Extending the routable address space required a breaking change and everyone recognized that it was likely to be the last such change for a very long time. This meant there was immense pressure to resolve any shortcomings in IPv4 and introduce new refinements to take advantage of this one, possible last, opportunity to make breaking changes.