The, apparently non-trivial, mechanism of acquiring a domain name from an IP address (the opposite of the usage of DNS).
Non-trivial partly because it usually involves convincing a different set of people to the ones that handle your DNS that you are who you say you are and own the IP addresses and domain names you say you do; these people typically have far less interest in doing the (relatively trivial) work involved than the ones who sold you the domain names; the work may be more tricky than it appears at first glance if the IP addresses you own are part of an unclassed block (or whatever the term is). - MoonShadow
The difficulty lies in delegating blocks of less than 256 IP addresses, and is actually not that difficult; the trick for doing it has been well-documented for many years now. It's called "classless" delegation because such blocks only became practical to route when classless routing was introduced.