To add to Onomatopoeia’s excellent post, separate devices also limit the blast radius of any compromise. Attackers pivot when they compromise a system. They use one system to talk to others and attack them from inside your network. So you don’t want everything on the same OS kernel.

Unfortunately I don’t feel like I’m qualified to say what works well yet, not until I have the pieces of my site put together and working, and vetted by whatever security professionals I can get to look at it and tell me what I did wrong.

But right now I think that looks like every service VM on its own VLAN on a /30 net, and ideally the service VM and firewall/router VM serving it on different physical hardware joined by a managed switch. That managed switch shouldn’t let either VM host touch its management VLAN, and (I think, I don’t do this yet) should send monitor traffic to yet another physical host for analysis.

(“I can see why you’re not done yet” - yeah I know.)