That's the same as having them within the same LAN. That makes https unnecessary, but you'd still have to remember ip+port combos without a domain. The domain doesn't need to be publicly registered unless you want services to be accessible externally without a VPN connection.

I only allow a couple of my services to be accessed directly via public domain, mostly for sharing with friends. The rest you've gotta be within the LAN either by wifi/ethernet or a VPN that I host.