Every computer that accesses the Internet is assigned an IP (Internet Protocol) address, which appears as four subsets ranging from 0 to 255 (example: 255.255.0.0) (See Reference 1) Theoretically, this number should identify any Internet user, but it isn't actually that simple. Due to the heavy traffic on the Internet, most Internet Service Providers (ISP) use dynamic (rotating) IP addresses rather than static (constant) ones. However, if someone is spamming you or harassing you on the web (usually by email), the IP address is useful to pinpoint the general location of the computer, and the owner of the address. If the IP address owner is an Internet Service Provider, you can contact them to help identify the harasser who is using their system.
Identifying an IP address
Open the email you wish to trace, and locate the email header - the detailed mix of code that gives the path the email took to get to you. Nowadays, the header is hidden by the email programs, but there is usually an option to open a "detailed header" or "advanced options," and this will usually show the full header.
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Identify the sender's IP address in the header. This is easier said than done because most headers show several IP addresses; but if an individual sent the email personally to you, the correct IP address is usually found in the last "Received: from" line. Copy the IP address to your clipboard.
Go to a website that identifies IP addresses, such as ws.arin.net or www.IP-address.com. Paste the IP address in the search bar and press "Submit Query." If the IP address is valid, owner information and location will show up on the next page. If you're fortunate enough to have found a static IP address, you will receive more detailed information about the sender; if it is a dynamic IP address, it will identify the Internet Service Provider. Contact the ISP to report the sender.
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You can also identify visitors to your web site or blog by IP address using certain website trackers like SiteMeter, which will give you a bit of code to insert into your web page. You can check these IP addresses the same way as those found in the email headers.
If you are being emailed by a professional spammer, chances are much of the information in the email header is faked, and it is more difficult to identify the true IP address. For these, it's recommended to use a service like SpamCop, which will analyze your email header, locate the IP address and report the spammer for you.