The Internet is a worldwide network of unrelated computers that can only communicate successfully by agreeing to incorporate a set of rules or protocols. One of the most basic tenets of the Internet is the Internet Protocol (IP) address.
An IP address refers to a unique number assigned to each computer on the Internet. The IP number is called an "address" because it serves the same purpose as a home address -- it allows each machine on the Internet to be located by a numerical scheme. A domain name, such as www.wiseGEEK.com, also maps to a numerical IP address. In this case, it is 126.96.36.199.
Most people are familiar with the idea of an IP address through getting an Internet Service Provider (ISP). The ISP might offer either a dynamic or static address. A static one is a permanent address, while a dynamic address changes with each new Internet session. Every ISP has a unique range of available IP addresses.
Unless one arranges for a static IP address, a dynamic address is automatically assigned in the login process and will persist until the connection is intentionally ended or otherwise disabled. Upon logging in again, a new one is assigned. In each case, machines on the Internet will accept requests and return data to the current IP address of your machine. This IP number becomes part of the "return address" on the "envelope" as the browser requests Web pages, mail, and other online data.
An IP address can be mapped back to a specific user by requesting the log records of the ISP. In the case of unlawful online behavior, for example, law enforcement might subpoena an ISP for the name, address, and other personal information of the customer that was assigned a specific IP address associated with the criminal activity in question. Some ISPs do not require a subpoena.
Websites typically track visitors by their IP addresses to see what pages are of interest. Identities can be linked to the address when registration and cookies come into play. A cookie can "tell the site" who the user is, despite a changing IP address. The site may have true or false information, depending on what the visitor has provided.
Though the Internet might feel anonymous, it is not an anonymous environment by default. The IP address is the first and last link to the end-user. In order to surf anonymously, many people choose to "hide" their address by using a website that substitutes its own IP address for the user's. Sites that provide anonymous surfing keep the user's address hidden from the Internet, while acting as an intermediary. Of course, the website providing this service still has a record of the end user's IP address.