(like 200 for "OK", 301 for "Moved Permanently") that a webserver returns in response to a request for a URL. The 404 response code should be returned for a file 'Not Found'."

Whenever a users puts in the URL of your website, the webserver that is hosting your website searches for the corresponding file for the URL. When it finds the file, the server responds with a 200 response code, along with the most frequent message, such as the HTML.

Google defines response code 404, stating that, Let's say that in the link to "Visit Google Apps" above, the link is broken because of a typing error when coding the page. Now when a user clicks "Visit Google Apps", the particular webpage/file isn't located by the webserver. The webserver should return a 404 response code, meaning 'Not Found'.

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