IP(s) => Hostname(s); Host/Domain name => IP; WhoIs

Since I started this service in October 1998, several superior tools have appeared, such as: Visual Route (click on 'LIVE DEMO')
Net World Map
Geektools Whois BW whois The Elephants' Toolbox

My simple gh (get hostname) tool can still be useful in some cases:
Enter IP address; IP address followed by a number; or host/domain name:
Whois: YES NO
 
The latest version 1.44 is here for download: gh. You can either install it on your own site as above, or use it at the command prompt on any computer connected to Internet (and with Perl installed).

List of Whois servers (if all the above fails): MIT.
An old one-way Dutch IP translator