The research involved researching a company which had recently emerged from Chapter 11. In looking at Google's content relating to the company I came across http://tanaya.net/dns/.
Try it out for yourself, using the first three octets of your company's IP space. Example: http://tanaya.net/dns/10/10/5.text.
Turns out this listing is supposedly generated by the Bulldog firewall. As the dates on the files are not that old, I'm assumed these are updated periodically. It looks like the entire IP space. Interesting that someone would put that much work into tracking something as fluid as the Internet.