You can continue using /etc/resolv.conf to specify the private IP addresses even for a website that does actually exist. The point of registering the domain is

You can continue using /etc/resolv.conf to specify the private IP addresses even for a website that does actually exist. The point of registering the domain is to ensure that your computer/software can't ever accidentally hit anything you didn't intentionally configure that is actually under the control of someone else. You don't actually need to use publicly visible DNS to configure your internal network just because you used a real domain name.