HTTP 501 – Not Implemented
The request method is not supported by the server and cannot be handled.
Common use case
Returned when a server receives a method it doesn't recognise or hasn't implemented at all, e.g. an obscure WebDAV verb hitting a plain HTTP server.
Example
HTTP/1.1 501 Not Implemented
History
Part of HTTP/1.0 (RFC 1945, 1996).
Did you know?
Unlike 405 (which means "this resource doesn't support that method"), 501 means the server itself has no concept of that method at all.