Ever have an IP address in your logs that you can't identify? No reverse DNS, ARIN/WHOIS come up useless, just can't figure it out?
Try opening a web browser and entering HTTPS://<IP address> into the address window. With a little luck you will get a certificate error (due to mismatch, URL is an IP address, cert will have host/domain name)- look at the certificate and you have a domain and company name. A little tedious, but very handy.