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
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.
404 errors should not be confused with DNS errors, which appear when the given URL refers to a server name
that does not exist. A 404 error indicates that the server itself was found, but that the server was
not able to retrieve the requested page.
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