4xx HTTP Status Messages Quick Reference

A 404 error on wikipedia's page
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4xx:

This class of HTTP status message indicates there is a problem or error at the client or user agent end.

400 Bad Request:

The request could not be understood by the server due to malformed syntax.

401 Unauthorized:

The request requires user authorization (such as through htaccess) but the authorization codes sent were invalid or the user was not recognized in the system. This error is sent when the username sent is not recognized and when the username and password combination are incorrect.

402 Payment Required:

This HTTP status code is not used but is reserved for future use.

403 Forbidden:

The server understood the request but refuses to fulfill it. Authorization, in this case, doesn't matter.

404 Not Found:

This is the most easily recognized error message. It states that the URI requested does not exist on the server.

405 Method Not Allowed:

The method specified is not allowed for the resource requested.

406 Not Acceptable:

The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.

407 Proxy Authentication Required:

This is similar to 401, but the client must first authenticate with a proxy.

408 Request Timeout:

The client did not produce a request within the time that the server was prepared to wait.

409 Conflict:

The request could not be completed due to a conflict with the current state of the resource.

410 Gone:

The resource requested was once on this server but is no longer here and there is no redirect in place for it.

411 Length Required:

The server requires a content-length sent with the request.

412 Precondition Failed:

The server evaluated a pre-condition in the header fields as false.

413 Request Entity Too Large:

The server has a limit as to the size of a request.

414 Request-URI Too Long:

The server has a limit as to the size of a URI.

415 Unsupported Media Type:

The entity of the request is a format not supported by the requested resource.

416 Requested Range Not Satisfiable:

A server should return a response with this status code if a request included a Range request-header field and none of the range-specifier values in this field overlap the current extent of the selected resource, and the request did not include an If-Range request-header field.