I am checking out a client's web address to ip. Can nslookup the web address and get the ip. However, when I do an nslookup to confirm the web address, DNS says it is non-existent.

What am I missing here?

I know the site it live, but does this sound like they are blocking ip-to-web address lookups (is that what is called as "reverse-DNS" lookup)?

Also - if that is blocked, is that a good security practice to do so? Sounds like it might, but not sure.