The HTTP status code 404 or "Not Found" indicates the server could not find the requested file or resource.
A 404 often means that the URL requested is invalid or points to a file that no longer exists (e.g. has been moved or deleted). It can also mean that web service method or other similar resource does not exist.
Many web servers can be configured to display a specific error page when this occurs which allows users to continue navigating in a manner consistent with the design of the website.
See RFC2616 for reference.
Microsoft's Internet Information Services (IIS) versions 7, 7.5, and 8.0 provide additional sub-status values for various types of 404 response statuses. These are not recognized as standard status codes, but may be relevant for IIS-hosted applications.
Microsoft Sub-Status Codes:
404.0 – Not found.
404.1 – Site Not Found.
404.2 – ISAPI or CGI restriction.
404.3 – MIME type restriction.
404.4 – No handler configured.
404.5 – Denied by request filtering configuration.
404.6 – Verb denied.
404.7 – File extension denied.
404.8 – Hidden namespace.
404.9 – File attribute hidden.
404.10 – Request header too long.
404.11 – Request contains double escape sequence.
404.12 – Request contains high-bit characters.
404.13 – Content length too large.
404.14 – Request URL too long.
404.15 – Query string too long.
404.16 – DAV request sent to the static file handler.
404.17 – Dynamic content mapped to the static file handler via a wildcard MIME mapping.
404.18 – Query string sequence denied.
404.19 – Denied by filtering rule.
404.20 – Too Many URL Segments.
