Although in theory, the entire content of the Web can be searched, several factors prevent this.

First, search engines do not always have access to all areas of a web site, such as information stored in a site's custom database. For example, Medline abstracts are stored and maintained in a custom database, which is used to construct temporary Web pages of search results.  Medline abstracts are only available through Medline searches, and routine Web search engines will not retrieve any content directly from Medline.

Second, Web content constantly changes, and sites may be intermittently unavailable to search engines.  The search engine may index and list a page which moves, changes, or is otherwise unavailable by the time it appears as a search result. Google (17) and a few other engines store web pages in a cache, so that if the page is unavailable, an archived copy may still be accessible. Alltheweb (3) reindexes frequently and is likely to have very current content.