Yes. 5xx codes are server errors: The server failed to fulfil a valid-looking request.

5xx · Server error

HTTP 511 Network Authentication Required

You must authenticate to gain network access, typical of captive portals such as hotel Wi-Fi.

Example response

HTTP

HTTP/1.1 511 Network Authentication Required
Content-Type: application/json; charset=utf-8

{ "error": "Network Authentication Required" }

Look up the headers shown here in the HTTP headers reference.

What does HTTP 511 mean?: Network Authentication Required. You must authenticate to gain network access, typical of captive portals such as hotel Wi-Fi.
Is 511 an error?: Yes. 5xx codes are server errors: The server failed to fulfil a valid-looking request.