I have a virtualmin box on datacenter. Next week I'll send another 3 virtualmin boxes to the datacenter. I asked the datacenter support what was the best way for the servers to access each other internally without the public IP, something like 192.168.100.1. All my servers have dual Ethernet connections, so they told me that the best way is to send a switch to the datacenter, and link each second Ethernet to the switch.
Ok. Question is, how do I properly configure the second Ethernet card to be used internally only and it doesn't interfere with the first one? I've tried to do that once, but the server could not access the internet anymore and all services became offline. So I removed the second Ethernet configuration to restore the services back online.