Search engine limitations

Many, probably most, of the publicly available web pages in existence are not indexed. Each search engine captures a different percentage of the total. Nobody can tell exactly what portion is captured.

The estimated size of the World Wide Web is at least 11.5 billion pages[2], but a much deeper (and larger) Web, estimated at over 3 trillion pages,[citation needed] exists within databases whose contents the search engines do not index. These dynamic web pages are formatted by a Web server when a user requests them and as such cannot be indexed by conventional search engines. The United States Patent and Trademark Office website is an example; although a search engine can find its main page, one can only search its database of individual patents by entering queries into the site itself.[3]

Google, as all search engines should, follows the robots.txt protocol and can be blocked by sites that do not wish their content to be indexed or cached by Google. Sites that contain large amounts of copyrighted content (Image galleries, subscription newspapers, webcomics, movies, video, help desks), usually involving membership, will block Google and other search engines. Other sites may also block Google due to the stress or bandwidth concerns on the server hosting the content.

Google has also been the victim of redirection exploits that may return more results for a specific search term than exist actual content pages.

Google and other popular search engines are also a target for search engine "search result enhancement", also known as search engine optimizers, so there may also be many results returned that lead to a page that only serves as an advertisement. Sometimes pages contain hundreds of keywords designed specifically to attract search engine users to that page, but in fact serve an advertisement instead of a page with content related to the keyword.

Search engines also might not be able to read links or metadata that normally requires a browser plugin, Adobe PDF,or Macromedia Flash, or where a website is displayed as part of an image. Search engines also can not listen to podcasts or other audio streams, or even video mentioning a search term.

Forums, membership-only and subscription-only sites (since Googlebot does not sign up for site access) and sites that cycle their content are not cached or indexed by any search engine. With more sites moving to AJAX/Web 2.0 designs, this limitation will become more prevalent as search engines only simulate following the links on a web page. AJAX page setups (like Google maps) dynamically return data based on realtime manipulation of javascript.