"IP exists, but does not resolve to a host name"

When I run a check on the IP of someone who is posting in my forum, this is the message I get. What does it mean?

It means that it can't find a lookup table that defines a domain name to correspond to the ip address.

Every domain name to be usable must be linked to an ip address but an ip address is usable even if there is no domain name linked to it. (Like if someone knows where you live they can visit you even if they don't know your address).

Is it leagal though? Or, is it something like a hacker or the like?

I get a lot of IP's that come up with no host name to them and such.

If they have posted in my guestbook or things I just delete there posts and or block their IP's.

Sometimes these come from ones with unknown browsers as well don't know on that part either.

But I usually take them as spammers or something else.

I myself don't want to be sitting all day blocking IP's and checking everyone one that signs my guestbook and things and more stuff if I know it is legit I keep it if phony or suspicous stuff I delete it and such things.

But one thing too with blocking IP's you could be blocking yourself in there or a friend or something inadvertently.

Thank you Stephen & ???