Ad filtering or ad blocking is a service which removes or alters advertising content in a Web page. This content can be represented in a variety of ways including pictures, animations, text, or pop-up windows. More advanced filters allow fine-grained control of advertisements through features like blacklists, whitelists, and regular expression filters. Certain security features also have the effect of disabling some ads.

The immediate benefits include cleaner looking Web pages free from advertisements and lower resource-usage (bandwidth, CPU, memory, etc.). One drawback is that advertisements are a major source of revenue for many websites. However, the actual loss of revenue, when present, is difficult to measure.

Browser integration

Almost all modern web browsers include a pop-up blocker. Opera and Konqueror also include content filtering, which prevents external files such as images or JavaScript files from loading. Content filtering can be added to Mozilla Firefox and related browsers with Adblock or Adblock Plus, and a number of sources provide regularly updated filter lists. For Internet Explorer there are several add-ons available like IE7Pro, Adblock Pro and Quero that also allows to temporarily unblock blocked content. A further method for filtering advertisements uses CSS rules to hide specific HTML and XHTML elements.

External programs

A number of external applications offer ad filtering as a primary or extra feature. A traditional solution is to customize an HTTP proxy (or web proxy) to filter content. These programs work by caching and filtering content before it is displayed in a user's browser. This provides an opportunity to remove not only ads but also content which may be offensive or inappropriate. Popular proxy software which block content effectively include Privoxy, Squid, Proximodo, Adextinguisher, Ad Muncher, Guidescope and Proxomitron. The main advandage of the method is freedom from implementation boundaries (browser, working techniques) and centralization of control (the proxy can be used by many users). The major drawback is that the proxy sees only raw content andthereforeit's difficult to handle JavaScript-generated content.

Hosts file

Further information: hosts file.

This method exploits the fact that most operating systems store a file with IP address, domain name pairs which is consulted by most browsers before using a DNS server to look up a domain name. By assigning the local 127.0.0.1 IP number to known ad servers the user directs traffic intended to reach those ad servers to the local machine. Running a suitable web server locally the ad content can be replaced with anything the user wishes. For instance if the web server sends a blank html page for any request the ads completely disappear from the pages they were originally intended to appear on .

DNS Filtering

Advertising can be blocked by using a DNS server which is configured to block access to domains or hostnames which are known to serve ads. At least two sites exist that can provide this service: OpenDNS and AdBarricade.

Common advertising techniques

Pop-up ads

Plain text

Ad banners

Flash animations

Keyword hyperlinks (for example Vibrant Media's IntelliTXT)

Browser plugins/extensions (often labeled as adware)

External applications (see adware, spyware)