- you would need to restrict yourself to an extremely tiny fraction of the v6 space that's the same size as the v4 space

- you couldn't use any IPs that correspond to in-use v4 IPs because they would overlap

- you couldn't use any IPs that correspond to unusable v4 IPs, for the same reasons you can't use them in v4

- you'd have to talk a protocol that looks the same as v4 on the wire, because otherwise v4 hosts won't be able to handle it

You know what we call that? We call it IPv4.