Alternatively, put the IPV4 block as part of the IPV8 [I'm making up a fictional logical profile]. For example, the address 100.101.102.103 in IPV4 could just be 0.0.0.0.100.101.102.103 in a new, expanded 64-bit address space. So you want the 128-bit address space? How about 0.0.0.0.0.0.0.0.0.0.0.0.100.101.102.103. IPV8 could be a drop-in replacement for IPV4, because anything that worked on IPV8 would coexist perfectly with IPV4 addresses. Anything that didn't work with IPV8 would still have all of the IPV4 address space while it gets updated.