An HTTP response contains a result code, header fields, and a body. The HTTP protocol expects the result code and all header fields to be returned before any body content.
Some commonly used status codes include the following:
404: Indicates that the requested resource is not available
401: Indicates that the request requires HTTP authentication
500: Indicates an error inside the HTTP server which prevented it from fulfilling the request
503: Indicates that the HTTP server is temporarily overloaded, and unable to handle the request