||This article may contain original research. (September 2012)|
Ad filtering or ad blocking is removing or altering advertising content in a webpage. Advertising can exist in a variety of forms including pictures, animations, embedded audio and video, text, or pop-up windows. Very often it employs autoplay of audio and video. It is a known problem with most web browsers, including Firefox, that restoring sessions often plays multiple embedded ads at once. All browsers offer some solution to the problem, either by targeting technologies (Flash/Shockwave, Window Media files, etc.) that are used to deliver ads, targeting URLs that are the source of ads, or targeting behavioural characteristic of ads (such as the use of HTML5 autoplay of both audio and video).
An extremely common method of filtering is simply to block (or prevent autoplay of) Flash animation or image loading or Windows audio and video files. This can be done in most browsers easily. See browser integration below. This crude technological method is refined by numerous browser extensions. See Firefox, Safari, Internet Explorer, Opera, Chrome for details specific to each. In general one alters the options, preferences or application extensions to filter specific media types, but an additional add-on is required to differentiate between ads and non-ads using the same technology, or between wanted and unwanted ads or behaviors.
The more advanced filters allow fine-grained control of advertisements through features such as blacklists, whitelists, and regular expression filters. Certain security features also have the effect of disabling some ads. Some antivirus software can act as ad blocker, including some freeware such as Avast!.
Ironically, some of this freeware itself runs ads, and instructions on how to block those are common on the web. For instance, Avast! has ads itself and must be upgraded to the paid version to disable them. 
Filtering by intermediaries such as providers or national governments is increasingly common. See below especially re provider ad substitution and national root DNS.
Benefits of ad filtering
To users, the benefits of ad blocking include quicker loading and cleaner looking Web pages free from advertisements, lower resource waste (bandwidth, CPU, memory, etc.), and privacy benefits gained through the exclusion of the tracking and profiling systems of ad delivery platforms. Blocking ads can also save minimal amounts of energy.
Users who pay for total transferred bandwidth ("capped" or pay-for-usage connections) including most mobile users worldwide, have a direct financial benefit from filtering an ad before it is loaded. Streaming audio and video, even if they are not presented to the user interface, can rapidly consume gigabytes of transfer especially on a faster 4G connection. In Canada, where users without a data plan often pay C$0.50/megabyte ($500/gigabyte) for at least the first 50-100MB exceeding their data allowance, the cost of tolerating ads can be intolerable. Even fixed connections are often subject to usage limits, especially the faster connections (100Mbit/s and up) which can quickly saturate a network if filled by streaming media. "The extent of unlimited bandwidth plans is often grossly over-estimated by US and European users and advertisers. This problem affects other countries, especially those with bandwidth limitations on their global Internet connections, or those that have poor regulatory or effective monopoly providers."[this quote needs a citation]
To advertisers, the benefits include not angering or annoying users into blocking, defaming or boycotting their products or websites. Few advertisers actually intend to anger end users. Very sophisticated filtering and anti-spam techniques can involve active defenses which can shut down an advertiser's domains or brokers, ban them from searches or target them for other countermeasures. Some countries have even considered banning the use of certain ports, e.g. South Korea's proposed ban on port 25 used by SMTP. Future countermeasures would be likely to include bans on ads South Koreans are unlikely to want or even ad brokering services. Ad substituting is also a legal and common practice already, for instance in Canadian cable TV where regulations permit showing a Canadian channel with Canadian ads instead of a US channel with US ads, where both are broadcasting the show simultaneously - this practice has spread to the web with some cable Internet providers uniformly substituting foreign ads, for local ones for which they receive a share of the revenue. Avoiding national, provider or technological interference with their ads is a priority for advertisers and especially brokers of advertising, to whom it could be fatal.
Economic consequences for online business
One consequence of widespread ad blocking is decreased revenue to a website sustained by advertisements, where this blocking can be detected.
A number of website operators, who use online advertisements to fund the hosting of their websites, argue that the use of ad-blocking software risks cutting off their revenue stream. While some websites have successfully implemented subscription and membership based systems for revenue, the majority of websites today rely on online advertising to function.
Advertiser offensive countermeasures and justifications
Some websites have taken counter-measures against ad-blocking software, such as attempting to detect the presence of ad blockers and informing users of their views, or outright preventing users from accessing the content unless they disable the ad-blocking software. There have been several arguments supporting and opposing the assertion that blocking ads is wrong.
- 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 loopback address to each known ad server, the user directs traffic intended to reach each ad server to the local machine. Running a suitable web server locally the ad content can be replaced with anything the user wishes.
This method operates by filtering and changing records of a DNS cache. On most operating systems the domain name resolution always goes via DNS cache. By changing records within the cache or preventing records from entering the cache, programs are allowed or prevented from accessing domain names. The external programs like Portable DNS Cache and Firewall  monitor internal DNS cache and import DNS records from a file. As a part of the domain name resolution process, a DNS cache lookup is performed before contacting a DNS server. Thus its records take precedence over DNS server queries. Unlike the method of modifying a Hosts file, this method is more flexible as it uses more comprehensive data available from DNS cache records.
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.
Morally, while some argue that domain name holders are owners of property (and have been found to have such rights in most developed countries), it has also been one of the web's most basic features that DNS can be localized and run on client, LAN, provider and national services. China, for instance, runs, its own root DNS and the EU has considered the same. Google has required their Google Public DNS be used for some applications on its Android devices. Accordingly, DNS addresses / domains used for advertising may be extremely vulnerable to a broad form of ad substitution whereby a domain that serves ads is entirely swapped out with one serving more local ads to some subset of users. This is especially likely in countries, notably Russia, India and China, where advertisers often refuse to pay for clicks or page views. DNS-level blocking of domains for non-commercial reasons is already common in China.
Ad filtering by external parties and internet providers
Internet providers, especially mobile operators frequently offer proxies designed to reduce network traffic. Even when not targeted at ad filtering specifically these will block many types of advertisements that are too large, bandwidth consuming or otherwise deemed unsuited for the specific internet connection or target device.
- Ad server
- Adversarial information retrieval
- Hosts file filtering
- Proxy server#Content filter
- Adblock Plus
- AdBlock (Chrome)
Common advertising techniques
- "Ad Blocking is Immoral | The Google Cache: Search Engine Marketing, SEO & PPC". The Google Cache. 2007-08-02. Retrieved 2011-11-04.
- "Adblock: Adapt, or die.Service Assurance Daily: Anything and everything that affects IT performance, from the mundane to the bizarre - Network Performance Blog". Networkperformancedaily.com. 2007-09-05. Retrieved 2011-11-04.
- Kirk, Jeremy (2007-08-23). "Firefox ad-blocker extension causes angst | Applications". InfoWorld. Retrieved 2011-11-04.
- Paul Thurrott. "IE8: Ad blocking with the InPrivate Filter". Retrieved 5-October-2009.
- Ad Blocking with DNS Filtering