When communicating via HTTP, a server is required to respond to a request, such as a web browser request for a web page, with a numeric response code and an optional, mandatory, or disallowed (based upon the status code) message. In the code 404, the first digit indicates a client error, such as a mistyped Uniform Resource Locator (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.
A 404 error is often returned when pages have been moved or deleted. In the first case, it is better to employ URL mapping or URL redirection by returning 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.