HTTP 501 status code means a server can’t process the request because it doesn’t support a particular feature.
The most common example of this would be sending an unsupported request method to the server.
This status code is similar to 405 Method Not Allowed. Semantically,
4xx are client errors, while
5xx are server errors. This implies that 405 is appropriate when a client shouldn’t have requested the particular method, while 501 means a server doesn’t understand the requested method.
Try it yourself
Send an arbitrary request method using
-X, --request option in curl:
curl --verbose -X LOL http://example.com
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