How Ad-Blocking Software Works
April 23, 2001
What happens when you load a web page?
What does a proxy server do?
A proxy server is a piece of software (perhaps on a separate computer) that you can funnel all of your browser's data requests through before they reach the Internet. Proxy servers can be used by companies to avoid the security risk of having all their computers connect directly to the Internet. They also allow frequently requested pages to be cached or access to certain sites to be restricted (stop browsing for porn at work!).
How does ad-blocking software work?
Most ad-blocking software installs a customized proxy server on your computer and instructs you (or does it for you) to change your browser settings to route HTTP requests through this proxy server.
At the simplest level, the ad-blocking software causes each request for data to be inspected to see if it is being sent to the URL of a known ad-server (to download a banner ad, for example). Instead of passing the request along to the ad-server (as the proxy server does for normal page requests), it can throw the request away and tell the browser that the requested data could not be found, or it could send a bogus image to the browser (for example, a single transparent pixel). Thus the ad request never makes it to the ad server and the browser does not display an ad.
Automatically Blocking Banner Ad URLs
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