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The 404 or Not Found error message is an HTTPstandard response code indicating that the client was able to communicate with the server, but the server either could not find the file that was requested, or was unwilling to fulfill the request for it and did not wish to reveal the reason why.
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.
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.
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; 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 is also a neologism denoting when a thing or person is not found.