Most data centers actively want the default private networking to work across the datacenter with everyone. A better term would be non-routing. Rackspace calls

Most data centers actively want the default private networking to work across the datacenter with everyone. A better term would be non-routing. Rackspace calls theirs "service net". These non-routing networks encourage people to use cheap internal bandwidth (rather than bounce of an expensive leased line just to come back in) and possibly offer a special service just to that data center's customers. There are tons of companies offering sql and nosql databases as a service, and any number of other offerings. Most cloud providers recommend you configure your firewall rules keeping in mind that other people could be on your internal network connection. This is really a security best-practice anyway.