A status code of 400 indicates that the server did not understand the request due to bad syntax.
'''401 - Unauthorized'''
A 401 status code indicates that before a resource can be accessed, the client must be authorised by the server.
'''402 - Payment Required'''
The 402 status code is not currently in use, being listed as "reserved for future use".
'''403 - Forbidden'''
A 403 status code indicates that the client cannot access the requested resource. That might mean that the wrong username and password were sent in the request, or that the permissions on the server do not allow what was being asked.
'''404 - Not Found'''
The best known of them all, the 404 status code indicates that the requested resource was not found at the URL given, and the server has no idea how long for.
'''405 - Method Not Allowed'''
A 405 status code is returned when the client has tried to use a request method that the server does not allow. Request methods that are allowed should be sent with the response (common request methods are POST and GET).
'''406 - Not Acceptable'''
The 406 status code means that, although the server understood and processed the request, the response is of a form the client cannot understand. A client sends, as part of a request, headers indicating what types of data it can use, and a 406 error is returned when the response is of a type not i that list.
'''407 - Proxy Authentication Required'''
The 407 status code is very similar to the 401 status code, and means that the client must be authorised by the proxy before the request can proceed.
'''408 - Request Timeout'''
A 408 status code means that the client did not produce a request quickly enough. A server is set to only wait a certain amount of time for responses from clients, and a 408 status code indicates that time has passed.
'''409 - Conflict'''
A 409 status code indicates that the server was unable to complete the request, often because a file would need to be editted, created or deleted, and that file cannot be editted, created or deleted.
'''410 - Gone'''
A 410 status code is the 404's lesser known cousin. It indicates that a resource has permanently gone (a 404 status code gives no indication if a resource has gine permanently or temporarily), and no new address is known for it.
'''411 - Length Required'''
The 411 status code occurs when a server refuses to process a request because a content length was not specified.
'''412 - Precondition Failed'''
A 412 status code indicates that one of the conditions the request was made under has failed.
'''413 - Request Entity Too Large'''
The 413 status code indicates that the request was larger than the server is able to handle, either due to physical constraints or to settings. Usually, this occurs when a file is sent using the POST method from a form, and the file is larger than the maximum size allowed in the server settings.
'''414 - Request-URI Too Long'''
The 414 status code indicates the the URL requested by the client was longer than it can process.
'''415 - Unsupported Media Type'''
A 415 status code is returned by a server to indicate that part of the request was in an unsupported format.
'''416 - Requested Range Not Satisfiable'''
A 416 status code indicates that the server was unable to fulfill the request. This may be, for example, because the client asked for the 800th-900th bytes of a document, but the document was only 200 bytes long.
'''417 - Expectation Failed'''
The 417 status code means that the server was unable to properly complete the request. One of the headers sent to the server, the "Expect" header, indicated an expectation the server could not meet.