What does the first digit mean?
HTTP responses carry a three-digit status. 1xx provides interim information, 2xx indicates success, 3xx concerns redirection or cache validation, 4xx indicates request-related problems and 5xx server-side failures. An HTTP error status demonstrates that an HTTP response arrived. DNS, connection or TLS failures may produce no status at all.
Success and redirection
200 is a successful response, 201 reports a newly created resource and 204 success without content. 301 and 308 are permanent redirects, while 302 and 307 are temporary. 307 and 308 preserve the request method; with 301/302 a client may change POST to GET. 304 supplies no new content because a conditional request validates the cached version.
Request-related errors
400 indicates a bad request. 401 requires valid authentication, while 403 refuses processing; the user need not already be logged in. 404 means the target provides no matching resource or declines to disclose its existence. 405 rejects the method. 429 indicates too many requests and may include Retry-After guidance.
Server errors and our test
500 is a general server error, 502 an invalid gateway response, 503 temporary unavailability and 504 a gateway timeout. Our availability checker requests the site from Düsseldorf and follows permitted redirects. Browsers may receive different responses because of cookies, login, method or location. A 200 status alone does not prove that the content works.