Mass IP Whois Lookup

Let's say you've had 10k visitors to your website. About 100 of them visited your website from their workplace. You want to contact the employer of the visitor but you only have the IP address. How would you do it?

What tool or command line would you use? What database? What's needed to filter out 10k IP addresses?

Thanks!

Unless the company/business whose IP you have has registered with ARIN you don't. A lot of big corporations are their own ISP basically, unless that's the case though you won't be able to locate them that easily. For instance if Johnny works at the local store and visits your site, chances are you got the stores IP, the only problem is, more than likely that IP is just a regular Comcast IP, or maybe a Comcast Business IP. Since most of the residential networks also offer business plans, you'll usually wind with something from like Road Runner, Comcast, Verizon, etc. Most ISP's won't release subscriber information w/o a subpoena, so that's why you'll run into problems.

Hm yeah, I get that, thanks. But if it is registered with ARIN or RIPE (?), which some are, then I can do it? What's the best way here?

A mass reverse DNS lookup would suffice already, I think? Get the unique domains, browse the site and find contact information?

http://www.maxmind.com would give you the ISP information. If you're just looking to market to these companies an easier way might be to use re-targeting advertising such as http://www.adroll.com or http://www.perfectaudience.com

Find the company by the browsing the ip or reverse ip address looking and then contact the employer using facebook graph search or linkedin. Google Analytics might come handy as well.
Sometimes scanning the ports of an ip-address helps too.