[Users] Simplified multilateral peering for a lab environment

Greetings, I've found some decent documentation on secure multilateral peering for OpenSER, however I'm looking for something more simple. In a lab environment, I'm looking to set up four simulated geographically diverse networks all peering across a common backbone. Each of the four networks should have its own SIP registrar. What I'm looking for is a quick guide on how to allow each of the four registrars tell the other three what prefixes are registered with what address so that all SIP UAs can dial just an extension to reach UAs in any of the four networks. Right now I'd like to leave things unsecured if possible (no need for TLS). Also, if there are any good pointers for setting up the DNS in each of the four networks (each network will have its own DNS as well as SIP server) those would be useful. Finally, IPv4 references would be helpful, but seeing as this will be a v6-only setup, any reverences to IPv6 configuration guides regarding the above would be especially useful. Thanks again, ~Aaron