The 4xx class of status code is indicated for cases in which the client seems to have errored.
400 (Bsd Request) - The request contains bad syntax or cannot be fulfiled.
403 (forbidden) - The request was a leagel one but the server is refusing to respond it.
404 (not found) - The requested resource could not be found out but may be availible in the future.
410 (gone) - The requested page is no longer availible at the server and no forwarding address is known. This condition is expected to be considered permanent. If the server does not know or has no facility to determine whether or not the condition is permanent, the status code 404 returns.
414 (requested url too long) - The requested url is too long for the server to process it.