Microsoft IIS (Internet Information Services) sometimes uses additional decimal sub-codes for more specific information, however, these sub-codes only appear in the response payload and in the documentation, not in the place of an actual HTTP status code.All HTTP response status codes are separated into five classes (or categories). The first digit of the status-code defines the class of response, the last two digits do not have any class or categorization role.
Status code 0 is a response object that generally means no response, and can occur for various reasons, like request time out. Status 0 is not a valid HTTP status code.
Status code 999 - a non-standard code is returned by some sites (e.g. LinkedIn) which do not permit scanning.