Online or free preferably.

I have 20 or so records that I need to find the domain names for.

Sorry, my original post has been deleted because I changed it too many times.

Do you want a whois lookup to say who owns the address/range or do you want to know which site/sites are hosted on it?

ping -a ?

nslookup ip-address
or
nslookup hostname

www.samspade.org

...or mail me a list, I am *that* bored today!

Sam Spade is very good for this kind of thing. Dead easy to use.

Chris

Thanks for all the responses, I used Sam Spade in the end.

Several records are still very confusing though, they resolve to RIPE server addresses and not registered domains?????

Very odd.

Not that I use it often, well several times a day

http://www.demon.net/external

You are relying on the fact that someone has set up a reverse dns lookup address thingame wotsit.
If they haven't then you will possibly get who owns that range of IP addresses.