When communicating via HTTP, a server is required to respond to a request, such as a web browser's request for an HTML document (web page), with a numeric response code and an email-like MIME message. In the code 404, the first "4" indicates a client error, such as a mistyped URL. The following two digits indicate the specific error encountered. HTTP's use of three-digit codes is similar to the use of such codes in earlier protocols such as FTP and NNTP.
At the HTTP level, a 404 response code is followed by a human-readable "reason phrase". The HTTP specification suggests the phrase "Not Found and many web servers by default issue an HTML page that includes both the 404 code and the "Not Found" phrase. Webservers can typically be configured to display a more natural description, a branded page or sometimes a search form, but the protocol level phrase, which is hidden from the user, is rarely customized.
Internet Explorer (before Internet Explorer 7), however, will not display custom pages unless they are larger than 512 bytes, opting to instead display a "friendly" error page. This default behaviour can be changed under Tools | Internet Options by clicking on the Advanced tab and un-checking the "Show friendly HTTP error messages" check box.
A 404 error is often returned when pages have been moved or deleted. In the first case, a better response is to return a 301 Moved Permanently response, which can be configured in most server configuration files, or through URL rewriting; in the second case, a 410 Gone should be returned. Because these two options require special server configuration, most websites do not make use of them.
404 errors should not be confused with DNS errors, which appear when the given URL refers to a webserver which does not exist. These are not 404 errors, which are always returned by a webserver.
Some proxy servers generate a 404 error when the remote host is not present, rather than returning lower level errors such as hostname lookup failing, or "connection refused". This can confuse programs that expect and act on specific responses -they can no longer easily distinguish between an absent web server and a missing web page on a web server that is present.
In July 2004, the UK telecom provider BT Group deployed the Cleanfeed content blocking system, which returns a 404 error to any request for content identified as illegal by the Internet Watch Foundation.