Tracking a User Over the Web with Cookies

Internet advertising agencies are among the few organizations that can track an individual’s use of the Web because they typically deliver advertisements to many different sites. Here’s how they use cookies to do that:

1. A user visits Web Site 1 and retrieves a Web page with a banner ad (marked here as 1111). Although the Web page comes from Web Site 1, the banner ad is retrieved from the advertiser’s Web server by an HTTP link on the page. When the user’s browser requests the advertisement in the link, the advertiser’s Web server sends both the advertisement and a unique cookie (given the value, in the diagram, of ABC123) to the user’s computer.

2. The same user then proceeds to Web Site 2. Web Site 2 has no way of knowing that the visitor has visited Web Site 1 and cannot retrieve any cookies that Web Site 1 may have placed on the user’s machine. However, a page on Web Site 2 contains a banner ad that is delivered by the same company that delivers ads for Web Site 1. When the user retrieves the banner ad (2222) from the Web server, which is the same server from which she retrieved the earlier banner ad (1111), the server requests any cookies that it previously placed on the user’s computer. That cookie identifies the user as ABC123, and an entry can be made in the advertiser’s database, indicating which Web sites the user has visited.