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

Since I started this service in October 1998, several superior tools have appeared:
Geektools Whois (Others listed here have been discontinued in the meantime)
My simple gh (get hostname) tool is 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 gh 1.47 (Apr 11, 2021) is here to download.
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).

If all the above fails, here are some lists of Whois servers:
  github.com (most up-to-date?)
  NirSoft (2016)
  ("Historical" MIT March list (2000))