The 404 or Not Found error message is a HTTP standard response code indicating that the client was able to communicate with the server but the server could not find what was requested.
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 and an optional, mandatory, or disallowed (based upon the status code) message. In the 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
At the HTTP level, a 404 response code is followed by a human-readable "reason phrase". The HTTP specification suggests the phrase "Not Found"[1] and many web servers by default issue an HTML page that includes both the 404 code and the "Not Found" phrase. Webservers can typically be configured to display a more natural description, a branded page or sometimes a search form, but the protocol level phrase, which is hidden from the user, is rarely customized.
