What?! Is that made public anywhere? I have not heard that before. Why don't they have an overlay? I would think this is a non-starter for many(most?) businesses.
I think it's pretty clearly explained in DigitalOcean's docs. They have a public interface and a private interface. The private interface is not Internet routable but is routable by all other instances in the datacenter. I agree that it is a non-starter for me, but it's definitely public.
