Clear gifs (also called web beacons,) are invisible graphics loaded on a page by a server tasked with tracking users as they browse any number of participating websites. Websites employing clear gifs usually get cash for doing so. Some networks of websites may want to pool their tracking data, and so create a common clear gif server of sorts.

I was asked to describe how to make a clear gif:

Generally speaking it's fairly easy to create one of your own. The server must begin streaming the image to the user, but never stop. Then the server knows how long the browser actually views the page. The server can also easily give the browser a cookie, this way the browser will uniquely identify itself to the server even if the user's isp changes his/her ip address.

This happens periodically on any isp that uses dynamic ip addressing. The cookie is also useful for discriminating between multiple computers on a network which all share the same ip address. Additionally if the user doesn't visit that website for a period of time and then visits it again, that web browser will still identify itself as it was told when it first received the cookie.

Note however that cookies are browser specific, and do expire at the time that a server tells them to. Moreover a user can delete his cookies, block cookies from specific websites, or block them entirely. Most users enable cookies by default because some websites require them.

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