CIA 2010 covert communication websites / Find missing hits in IP ranges

All IP ranges have some holes in them for which we don't have a domain name.

It is because there was nothing there, or just because we don't have a good enough reverse IP database?

It is possible that DomainTools could help with a more complete database, but its access is extremely expensive and out of reach at the moment.

Censys is another option that would be good to try.