HTTP 40x -- Client Error Codes (IIS 6.0)
HTTP status codes in the 4xx range indicate that a problem occurred with the request. Request problems can range from the request not meeting authentication requirements to the request being malformed. RFC 2616, "Hypertext Transfer Protocol — HTTP/1.1", refers to request problems as client errors because the requests originate from the client. For more information about RFC 2616, see the W3C World Wide Web Consortium Web site.
Table 11.4 HTTP 400-Class Client Error Codes Returned by IIS lists the client error codes that IIS returns. The descriptions for most codes are self-explanatory. When additional information about a code is required, it is provided in one of the following sections.