A company might buy a portion of IP space, and that might be information you could find in WHOIS. Also, there might be reverse DNS on the IPs that the company uses, so it's merely a matter of looking at the host name.
And on the topic subject, although it's old, an IP alone cannot be used to trace the location of the IP's user. It must be combined with ISP subscriber information, which is generally available only to law enforcement. Many people don't keep tabs on where their personal information is available though, so it is possible to trace quite a few people starting with just their IP address. It's something I've done myself, and it's very easy to do if you understand a little human psychology and know the resources available.
If you are a writer, then the more you write, the better people can identify you. Different people have very distinct writing styles and mannerisms, so it is very easy to correlate pieces of writing. The Internet is a big place though, so you obviously need some leads.
It is possible to achieve 100% technical anonymity by using Tor. You must properly secure your computer setup as well, however, as many programs can leak traceable information.