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Q. How do websites know I’m using an ad blocker with my browser?
A. Ad-blocking extensions for desktop and mobile systems detect the advertisem*nts a website is trying to display and stop the browser from downloading them. The software has become more popular as sites have turned to increased advertising as a means of financial support. A recent report estimated that 11 percent of internet users used an ad-blocking tool last year to screen out advertisem*nts, speed up browsing and possibly thwart malware infection from compromised ads.
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In response, many sites that rely on ad revenue to cover some of their costs have begun to put up blocking pages that prevent you from seeing the story you want to read unless you disable the ad-blocking program. A site can detect the presence of an ad blocker when its ad-server resources are rejected or hidden when it tries to download the content. Some sites sniff out the software by downloading a small bit of JavaScript code meant to trigger an ad blocker, and then checking to see if the code actually made it through to the browser.
Depending on your software, blocking advertisem*nts may not have to be an all-or-nothing exercise. Most programs have settings to allow advertisem*nts to come through for selected sites, but block them on others. AdBlock, a popular filtering extension, has a guide to whitelisting specific sites so you can financially support them. Some sites, when sensing an ad blocker in the mix, may also ask you to turn it off when visiting in exchange for a lighter dose of advertising on the page.
As the use of ad blockers increases, some websites are going to stronger measures to stay in business. Some offer subscriptions for an ad-free experience or to see the site’s full content, and some may have a donation button displayed prominently on the page.
Personal Tech invites questions about computer-based technology to techtip@nytimes.com. This column will answer questions of general interest, but letters cannot be answered individually.
A version of this article appears in print on , Section
B
, Page
6
of the New York edition
with the headline:
How a Site Knows You’re Blocking Ads. Order Reprints | Today’s Paper | Subscribe
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