Re: IPv4 Address Pool Has Been Expanded Significantly
60 bits is 32+28 bits. 2^32 is ~4 billion and 2^28 is ~256 million.
The "Kosher" definition of a product / system that is backwards compatible to its predecessor is that upon introduction, everyone (from operator to end-user) can continue what each has been doing previously.
By this definition, v6 is definitely backwards compatible. Adding v6 support to a network, router or host has no impact on its existing v4 support, and you can continue using v4 as you were previously. So it meets this definition.