It depends on the person, as a sample size of one I was on 5,000IU and my levels were still on the low end (almost still deficient), and my calcium levels were still safely on the low side as well. Ultimately people should be getting their levels checked before and after to see exactly what the effect on them is
As others have mentioned there already is the ".home.arpa" TLD but I definitely think ".internal" is a step up in terms of clarity. That said, for my internal network I just put things under a subdomain of a domain I own so I can use HTTPS with a proper SSL cert