[p1mrx]

You're describing 6to4, where the existing IPv4 address space becomes 2002:nnnn:nnnn::/48. You can treat the 80 bit suffix as 8 bits when designing a network.

Problem is, stacking the new protocol on top of IPv4 was never very reliable, so 6to4 is mostly dead now. It would've worked a bit better if the Internet had used 2002::/16 exclusively.