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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.
Some websites report a "not found" error by returning a standard web page with a "200 OK" response code, falsely reporting that the page loaded properly; this is known as a soft 404.
Soft 404s are problematic for automated methods of discovering whether a link is broken.
This can confuse programs that expect and act on specific responses, as they can no longer easily distinguish between an absent web server and a missing web page on a web server that is present.
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 protocol level phrase, which is hidden from the user, is rarely customized.
Internet Explorer, however, will not display custom pages unless they are larger than 512 bytes, opting instead to display a "friendly" error page.
At the HTTP level, a 404 response code is followed by a human-readable "reason phrase".