A "Best Answer" on Yahoo Answers:
How to Trace an IP Address
Whenever you get online, your computer is assigned an IP address. An IP address is the Internet Protocol (IP) address given to every computer connected to the Internet. An IP address is needed to send information, much like a street address or P.O. box is needed to receive regular mail. Tracing an IP address is actually pretty straightforward, and even though it's not always possible to track down a specific individual, you can get enough information to take action and file a complaint.
[edit] StepsFind the IP number you wish to check. The format of an IP address is numeric, written as four numbers separated by periods. Each number can be zero to 255.
To find the IP of an e-mail sent to you, investigate the message's "headers" by using your e-mail program's "details" or "properties" function. For example, in Outlook Express, select "Properties" from the "File" Menu, or just press ALT+Enter. Next, select the "Details" tab. In Yahoo, click "Full headers" on the upper right hand corner while you see the message. In Hotmail, go to "Mail Display Settings" and set "Message Headers" to "Full" or "Advanced". See this website for more information on viewing headers.
Go to a website that will allow you to look up IP address information. See External Links below. Type the IP number in the input box and submit.
Understand that in many situations you will learn a few things about the IP address:
Which internet service provider (ISP) the user is using. In some cases this may be the user's company (e.g. Ford.com). In other cases it may be just one of the large ISPs such as ATT or Comcast.
The approximate physical location of the user (e.g. Palo Alto, California.)
Recognize that usually you will not learn the actual name of the person doing at that IP address (e.g. Joe Smith). ISPs will typically only release such information under a court order.
TipsAnother way--
if you are using a Windows computer--to find out some information about someone is to go to Command Prompt (if you do not know how to do that, go to the Start menu, Run, and type "cmd" and press OK) and type in tracert <IP address>. On UNIX (including Linux and OS X), open a shell and use the command traceroute <IP address>
Warnings
It is not always possible to find out someone's exact address from this. People can take extreme measures to remain anonymous (even forging e-mails). Your search may just result in knowing the company that the person gets their Internet connection from. You can report abuse to that company, but it's not likely they'll divulge contact information for the individual without a court order.
If you are being harassed or you are concerned about your safety, file a complaint with your local police authorities. If you traced the IP address through an e-mail, give them a copy of the e-mail with detailed headers showing.
