Das Protokoll HTTP nutzt Statuscodes zur Rückmeldung. Am bekanntesten ist wohl der "HTTP Error 404". 404 steht für "Datei nicht gefunden.
100 - Continue
101 - Switching Protocols
200 - OK
201 - Created
202 - Accepted
203 - Non-Authoritative Information
204 - No Content
205 - Reset Content
206 - Partial Content
300 - Multiple Choices
301 - Moved Permanently
302 - Found
303 - See Other
304 - Not Modified (Datei nicht geändert)
305 - Use Proxy
307 - Temporary Redirect
400 - Bad Request
401 - Unauthorized
402 - Payment Required
403 - Forbidden
404 - Not Found (Datei nicht gefunden)
405 - Method Not Allowed
406 - Not Acceptable
407 - Proxy Authentication Required
408 - Request Time-out
409 - Conflict
410 - Gone
411 - Length Required
412 - Precondition Failed
413 - Request Entity Too Large
414 - Request-URI Too Large
415 - Unsupported Media Type
416 - Requested range not satisfiable
417 - Expectation Failed
500 - Internal Server Error (Fehler im CGI-Script)
501 - Not Implemented
502 - Bad Gateway
503 - Service Unavailable
504 - Gateway Time-out
505 - HTTP Version not supported
Quelle: RFC 2616