You might want to learn about how IP works before you tell people what it can do. IP has on each packet both the source and the destination address, because th

You might want to learn about how IP works before you tell people what it can do. IP has on each packet both the source and the destination address, because the destination uses the source address to send back the response. Nothing works if you can't send responses. If your device has an address from the "extended address space", you cannot talk to any device that doesn't know about that "extended address space" because they won't know how to read your source address and how to send a response back to it. Just as with IPv6: You cannot talk to an IPv4-only device if all you have is IPv6. Essentially, IPv6 does exactly what you are suggesting. Things just don't work as you imagine them to, but rather one unavoidably ends up with "a completely separate network" if one does things as you suggest.