What, if anything, does it mean when you look up an IP and get told that "hostname such-and-such does not exist"? I started out assuming there was something hinky about them, and probably put "Watch This Address" flags on some perfectly innocent people.
Then, last time I played with my logs, I found a visitor to one of my very, very specialized pages-- the ones where I have to satisfy my curiosity by looking them up.
[Could be forged: hostname squiggle-29-22.domain.dotdot does not exist]
In this case, domain.dotdot happens to be a respectable foreign government.* Clueless when it comes to Internet-related stuff, but otherwise respectable. And I've had several other visits from the aaa sector. They're in the appropriate geographical region. Absolutely, beyond question, legit.
So what does "does not exist" mean?
Punchline: The unknown visitor came from a search engine, where they were looking for information on... Well, let's say it was the equivalent of someone from Whitehall googling "precedence in the House of Lords". I can only hope it's one of those countries where a .gov address doesn't necessarily mean you work for the government.
* On another occasion, I had a long visit from someone whose IP turned out to belong to my state's income-tax department. I could never figure out whether the state was really, seriously scraping the barrel... or whether someone whose salary I pay was spending their time in an inappropriate manner.