I don't think he's advocating that you should have unprotected internal networks; just that the naming scheme shouldn't be confidential, since it is one of the

I don't think he's advocating that you should have unprotected internal networks; just that the naming scheme shouldn't be confidential, since it is one of the first things that will be exposed in a compromise. Knowing the name "main.prd.example.com" doesn't help if it's got a bastion host, thorough firewall rules, key-only SSH login, et cetera.