RFC1918 will likely not work since NAT'd users will be using ZeroTier. But there is a solution to this EXACT problem. RFC6598! http://tools.ietf.org/html/rfc65

RFC1918 will likely not work since NAT'd users will be using ZeroTier. But there is a solution to this EXACT problem. RFC6598! http://tools.ietf.org/html/rfc6598 http://tools.ietf.org/html/rfc6598 I think they could safely use 100.64.0.0/10 -- It's arguably designed for this kind of thing.