I get asked why IPv6 is not backward compatible with IPv4. The question misstates the real issue: IPv4 is not forward compatible with anything. We made the same mistake with IPv6, if it was a mistake; there is no easy way to replace it with a new protocol by tweaking the old. However, at the point that we use up the address space in IPv6, I suspect that requirements will have changed and we will need something dramatically different. If that is true, it’s unlikely that we could predict now what that “something new” would be. So, yes, at the point that we run out of IPv6 addresses a few thousand years from now, we will need a new protocol. But by then, I expect we will have already replaced IPv6 for other reasons entirely.
True in every word, except that I think that IPv6 will not make it a few thousand years. If we think of ”The Internet of Things” IPv6 depletion will go much faster than we expect now. Just as we could not predict the speed of IPv4 depletion when we designed it.