In communication between browsers and servers there are some messages that might be returned from the server depending if the request was successful or not. They are part of the HyperText Transfer Protocol (HTTP) response status codes and can be standardized within IETF or non-standardized messages that are sometimes served by third parties.

The list below includes the most common status messages seen on Internet (without non-standardized ones like RFC, Microsoft, Twitter...).

INFORMATIONAL
MESSAGE DESCRIPTION
100 Continue The server has received the request headers, and the client should proceed to send the request body
101 Switching Protocols The requester has asked the server to switch protocols
103 Checkpoint Used in the requests proposal to resume aborted PUT or POST requests

SUCCESS
MESSAGE DESCRIPTION
200 OK The request was successful (standard message for successful request)
201 Created The request has been fulfilled, and a new resource is created
202 Accepted The request has been accepted for processing, but the processing has not been completed
203 Non-Authoritative Information The request has been successfully processed, but is returning information that may be from another source
204 No Content The request has been successfully processed, but is not returning any content
205 Reset Content The request has been successfully processed, but is not returning any content (document view has to be reset by the requester)
206 Partial Content The server is delivering only part of the resource due to a range header sent by the client

REDIRECTION
MESSAGE DESCRIPTION
300 Multiple Choices It offers a list of multiple choices that the client may follow (maximum five addresses)
301 Moved Permanently The requested page has moved to a new URI
302 Found The requested page has moved temporarily to a new URI
303 See Other The requested page can be found under a different URI
304 Not Modified Indicates the resourced URI has not been modified since last requested
306 Switch Proxy Specified proxy should be used by requester (message NOT USED anymore)
307 Temporary Redirect The requested page has moved temporarily to a new URI (difference from 302 is that the request method should not be changed when reissuing the original request)
308 Permanent Redirect The request and all future ones should be repeated using another URI

CLIENT ERROR
MESSAGE DESCRIPTION
400 Bad Request The request cannot be fulfilled due to bad syntax
401 Unauthorized Authentication is required (if possible) but has failed or not yet been provided
402 Payment Required Currently reserved for future use. Intended to be used for Internet payments
403 Forbidden The request was a legal request, but the server is refusing to respond to it
404 Not Found The requested page could not be found but may be available again in the future
405 Method Not Allowed A request method is not supported by that page (i.e. GET method used on a form requiring POST method)
406 Not Acceptable The server can only generate a response that is not accepted by the client
407 Proxy Authentication Required The client must first authenticate itself with the proxy
408 Request Timeout The server timed out waiting for the request
409 Conflict The request could not be processed because of a conflict in the request
410 Gone The requested page is no longer available and should be prohibited from future requests
411 Length Required The content's length is not defined. The server will not accept the request without it
412 Precondition Failed The server didn't meet one of the pre-conditions requested by the requester
413 Request Entity Too Large The server will not accept the request because the request entity is too large
414 Request-URI Too Long The server will not accept the request because the URI is too long.
415 Unsupported Media Type The server will not accept the request because the media type is not supported
416 Requested Range Not Satisfiable The client has asked for a portion of the file, but the server cannot supply that portion
417 Expectation Failed The server cannot meet the requirements of the Expect request-header field

SERVER ERROR
MESSAGE DESCRIPTION
500 Internal Server Error A generic error message, given when no more specific message is suitable
501 Not Implemented The server either does not recognize the request method or it lacks the ability to fulfill the request
502 Bad Gateway The server was acting as a gateway or proxy and received an invalid response from the upstream server
503 Service Unavailable The server is currently unavailable (overloaded or down)
504 Gateway Timeout The server was acting as a gateway or proxy and did not receive a timely response from the upstream server
505 HTTP Version Not Supported The server does not support the HTTP protocol version used in the request
511 Network Authentication Required The client needs to authenticate to gain network access