It's true that IP itself has a significant problem with devices that roam between networks. I believe that there were some attempts to get a solution into IPv6, but they were abandoned (sadly - that could have perhaps been the killer feature that would have made adoption a much easier sell).

I don't think you're right about neighbors though. IP does support broadcast to allow you to communicate with nearby machines. Of course, in real networks, this is often disabled beyond some small perimeters because of concerns for overwhelming the bandwidth.