@lain so this setup is, simply, two boxes with a cable between them. Couldn't be simpler.

Attempting to communicate between them using self-assigned addresses does not even work the majority of the time. For some or no reason, it just doesn't work, then for a minute or so it will work, then stop again. I can start ping and it will say unreachable for a few minutes, then start pinging successfully, then stop, with no intervention.