The 404 or Not Found error message, as commonly encountered when using a web browser on the World Wide Web, is a manifestation of one of the standard response codes of the Web's principal communication protocol, HTTP.
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, sometimes followed by an email-like MIME message. Each response code has an associated string of English text that must also be present. Response code 404's associated string is "Not Found". When sending a 404 response, web servers usually include in the response message a short HTML document that mentions both the numeric code and this string. Many browsers opt to show this message to the user, while some browsers instead display a "friendlier" error message.
A 404 response indicates that the web browser was able to communicate with the server, but the server either could not find what was requested, or was unwilling to fulfill the request for it and did not wish to reveal the reason why.
HTTP response codes have a specific interpretation. In 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.
These errors can be customized on a large number of such servers to display a page that could be of more help than a default. For example, this can be achieved in Apache by placing a .htaccess file on the Web server. Internet Explorer will not display these pages, however, unless they are larger than 512 bytes. Creating humorous 404 pages has become popular and websites (see external links) have been created for the sole purpose of linking to numerous amusing 404 error pages.
404 is also a neologism denoting when a thing or person is not found.
- Customizing a 404 error page - instructions on configuring a server to serve a custom 404 error.
- 404 Research Lab - Various 404 error pages
- room404.com - A site that purports to contain the origin of the actual number used as error message. Be patient while it loads, it takes approximately 10 - 20 seconds.fr:Erreur HTTP 404