What is 404?

An HTTP 404 Error Message is a standard webservers response when it cannot find the file requested by the user agent. 404 Error Messages are typically the result of misspelled URL's and pages being moved or removed. Misspelled URL's can be the result of either the end user incorrectly typing a URL into the address bar of his web browser, or the result of incorrect or outdated URL's in webpage links. While the 3xx series of error messages were designed to indicate proper procedure for handling moved and deleted webpages, few webmasters actually use them. HTTP 404 Error Messages are additionally used when the server determines that the user agent is not authorised to view content, but would prefer not to disclose the reason for denial.

The 4xx series of error messages indicate suspected client-side errors, therefore 404 errors are assumed to be end-user typos. In fact, this was primarily the reason for file not found errors in the early World Wide Web, as 'link rot' had yet to develop and only professionals were producing webpages at the time. Today, most webpages are accessed via search engines such as Google, and most 404 errors are the result of outdated links. Thus, many incidents leading to 404 errors are not client-side errors as the 4xx error series indicates, rather server errors (resulting from bad webmaster practices) that would be better described by the 5xx series of error messages (server error). Although the webserver can determine if the incorrect URL is the result of the address being typed in incorrectly or because of an incorrect or outdated link, the distinction is rarely made.

There are several steps that you can take when presented with a 404 error. If you had typed the URL into your browser, then double check the spelling. Be especially careful about capitalization, as Internet addresses are case sensitive. If you had clicked a link and gotten a 404 error, then check for obvious misspellings in the URL. Webmasters are (usually) human, and prone to making mistakes. Sometimes you can catch these. In any case, write to the webmasters of both the referring page and the 404 page. The referring webmaster should update or remove his link, and the target page should have a 3xx series error page to redirect traffic.