IP-address reverse lookup: Who's behind an IP address?

Clients and friends have asked me, on different occasions: Can I / How do I perform a reverse look-up from an IP address. In other words, Can I know who's behind an IP address? Sometimes this comes from curiosity (e.g.: analysing a web server log file), or sometimes from attempting to control abuse / spam on other systems. Since this seems to be a common question, here go my two googable cents...

The short answer is, yes, you can. One of the simplest ways to do a reverse look-up on an IP address is by using the whois service from ARIN (American Registry for Internet Addresses). Go to:

http://ws.arin.net/whois/

Enter the IP address and hit the Submit Query button. Depending on the IP address you have entered, you will get a more precise or less precise answer. Sometimes you might get, for example, a company name (meaning that IP address belongs to the company network), or an university name (meaning the IP address belongs to the university network), or an internet provider which provides access to several consumers (less detail). In any case, this is the starting point to figure out a first contact, then if you need so, you could try figuring out some more information by contacting the listed entity.