What does an ad blocker do?
An ad blocker is usually a browser extension that sits between the page and the internet. It does not remove ads after they appear. It stops your browser from downloading them in the first place.
It works from filter lists, long community-maintained lists of the addresses that serve ads and trackers. Every time a page asks for something, the blocker checks the address against those lists and quietly drops anything that matches.
Pages load faster, use less data and carry far fewer trackers. The trade-off is that the occasional site breaks, and some ask you to turn the blocker off.
One article can pull in content from several servers at once. Tap the laptop to switch the ad blocker on and watch which requests still get through.
How ad blockers work
Step by step
The page asks for its pieces
A web page is really a list of instructions: fetch this text, this photo, this script, this ad. Your browser works through that list and sends a separate request for each piece, often dozens of them.
The blocker checks every request
Before each request leaves your browser, the ad blocker compares its address with its filter lists. The article's own text and photos pass straight through, while addresses that belong to known ad and tracking servers match a rule.
Matching requests never load
A blocked request is simply cancelled before it leaves your device. The ad's images, video and scripts are never downloaded, which is why pages often load faster and use less data with a blocker on.
Leftover gaps get tidied up
Sometimes an empty box is left where the ad would have been. Blockers also carry cosmetic rules that hide those leftover boxes, so the page closes up neatly around the article.
Where the blocking can happen
- Browser extensions
- Add-ons like uBlock Origin and AdGuard that run inside one browser. They are the most flexible, and the easiest to switch off for a single site.
- Built-in blockers
- Some browsers, such as Brave, block ads by default, and Safari lets apps supply blocking rules for it.
- DNS-level blockers
- Services like Pi-hole or NextDNS refuse to look up ad domains for every device on your network, phones and smart TVs included.
- Filter lists
- The shared rulebooks behind most blockers, such as EasyList for ads and EasyPrivacy for trackers, kept up to date by volunteers.
What an ad blocker does and does not do
An ad blocker does
- Stop most ads and trackers from downloading
- Make many pages load faster and use less data
- Cut down on malicious ads that try to push scams or malware
An ad blocker does not
- Hide your browsing from your ISP
- Catch ads a site serves from its own domain, like sponsored posts
- Work on every site, since some detect it and ask you to switch it off
An ad blocker is a bouncer for your browser's requests. It checks every address against a list and turns away known ad and tracker servers, so their content never reaches you. It will not hide your browsing from your ISP or catch ads a site serves itself, and some sites will ask you to switch it off, but for everyday browsing it makes the web noticeably faster and quieter.
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