The HTTP501 Not Implementedserver error response status code means that the server does not support the functionality required to fulfill the request.

A response with this status may also include aRetry-After header, telling the client that they can retry the request after the specified time has elapsed.A501 response is cacheable by default unless caching headers instruct otherwise.

501 is the appropriate response when the server does not recognize the request method and is incapable of supporting it for any resource.Servers are required to supportGET andHEAD, and therefore must not return501 in response to requests with these methods.If the server does recognize the method, but intentionally does not allow it, the appropriate response is405 Method Not Allowed.

If you have visited a web page and you are seeing501 errors, these issues require investigation and fixing by server owners or administrators.You can clear your browser cache for the domain, disable proxies if you are using one, or try again later to see if it works as expected.

A501 response can occur if proxies cannot not handle request methods used in the context of HTTP Extension Framework (RFC 2774) applications.This status can also occur in Web Distributed Authoring and Versioning (WebDAV) when a request method (SEARCH,PROPFIND) does not have a URL handler configured to process it.

Status

501 Not Implemented

Examples

Extension method not supported

In the following HTTP Extension Framework example, a client sends a request with a mandatory extension specified in theC-MAN header.TheConnection header specifies that these extensions are to be handled on ahop-by-hop basis.A proxy refuses to forward theM-GET method, and sends a501 error in response:

M-GET /document HTTP/1.1Host: example.comC-Man: "http://www.example.org/"Connection: C-Man

HTTP/1.1 501 Not Implemented

Specifications

See also

• HTTP response status codes
• 510 Not Extended
• HTTP 501 errors in Microsoft ASP.NET documentation