Server Responses and Status Codes (Webmaster in a Nutshell, 3rd Edition)

17.2. Server Responses and Status Codes

The server's response to a client request is grouped into three parts. The first line is the server response line, which contains the HTTP version number, a three-digit number indicating the status of the request (called a server response code or status code), and a short phrase describing the status. The response line is followed by the header information and an entity body if there is one.

Status codes are typically generated by web servers. However, they can also be generated by CGI scripts that bypass the server's precooked headers and supply their own. Status codes are grouped as follows.

Code range	Response meaning
100-199	Informational
200-299	Client request successful
300-399	Client request redirected, further action necessary
400-499	Client request incomplete
500-599	Server errors

HTTP defines only a few specific codes in each range, although servers may define their own as needed. If a client receives a code it does not recognize, it should understand its basic meaning from its numerical range. While most web browsers handle codes in the 100-, 200-, and 300-range silently, some error codes in the 400- and 500-range are commonly reported back to the user (e.g., "404 Not Found").

17.2.1. Informational

A response in the range of 100-199 is informational, indicating that the client's request was received and is being processed.

100 Continue

17.2.2. Client Request Successful

A response in the range of 200-299 means that the client's request was successful.

200 OK: The client's request was successful, and the server's response contains the requested data.
201 Created: This status code is used whenever a new URI is created. With this result code, the Location header is given by the server to specify where the new data was placed.
202 Accepted: The request was accepted but not immediately acted upon. More information about the transaction may be given in the entity body of the server's response. Note that there is no guarantee that the server will actually honor the request, even though it may seem like a legitimate request at the time of acceptance.
203 Non-Authoritative Information: The information in the entity header is from a local or third-party copy and not from the original server.
204 No Content: A status code and header are given in the response, but there is no entity body in the reply. On receiving this response, browsers should not update their document view. This is a useful code for an imagemap handler to return when the user clicks on useless or blank areas of an image.
205 Reset Content: The browser should clear the form used for this transaction for additional input. Appropriate for data-entry CGI applications.
206 Partial Content: The server is returning partial data of the size requested. Used in response to a request specifying a Range header. The server must specify the range included in the response with the Content-Range header.

17.2.3. Redirection

A response code in the 300-399 range indicates that the request was not performed, and the client needs to take further action for a successful request.

300 Multiple Choices: The requested URI refers to more than one resource. For example, the URI could refer to a document that has been translated into many languages. The entity body returned by the server could have a list of more specific data about how to choose the correct resource.
301 Moved Permanently: The requested URI is no longer used by the server, and the operation specified in the request was not performed. The new location for the requested document is specified in the Location header. All future requests for the document should use the new URI.
302 Found: The requested URI temporarily has a new URI. The Location header points to the new location. If this is in response to a GET or a HEAD, the client should use the new URI to resolve the request immediately after receiving the response.
303 See Other: The requested URI can be found at a different URI (specified in the Location header) and should be retrieved by a GET on that resource.
304 Not Modified: This is the response code to an If-Modified-Since header, where the URI has not been modified since the specified date. The entity body is not sent, and the client should use its own local copy.
305 Use Proxy: The requested URI must be accessed through the proxy in the Location header.
307 Temporary Redirect: The requested URI has moved temporarily. The Location header points to the new location. Immediately after receiving this status code, the client should use the new URI to resolve the request, but the old URI should be used for all future requests.

17.2.4. Client Request Incomplete

A response code in the range of 400-499 means that the client's request was incomplete and may indicate further information is required from the client.

400 Bad Request: This response code indicates that the server detected a syntax error in the client's request.
401 Unauthorized

17.2.5. Server Errors

Response codes in the range of 500-599 indicate that the server encountered an error and may be unable to perform the client's request.

500 Internal Server Error