What I'm suggesting is both Alice and Bob speaking ipv6. ipv4 is only used for handing out assignments to ipv6 users. If most dual stack networks use the same assignments for ipv6 and ipv4, and ipv6 routing tables are directly copied from ipv4 routing tables, then Bob can talk to Alice if their ISPs support ipv6. It's a simple upgrade instead of having to get totally separate assignments.