“The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network (e.g. "captive portals" used to require agreement to Terms of Service before granting full Internet access via a Wi-Fi hotspot). Proposed in an Internet-Draft.[19]“ — wikipedia
“The client needs to authenticate to gain network access.
The response representation SHOULD contain a link to a resource that allows the user to submit credentials (e.g., with an HTML form).
Note that the 511 response SHOULD NOT contain a challenge or the login interface itself, because browsers would show the login interface as being associated with the originally requested URL, which may cause confusion.
The 511 status SHOULD NOT be generated by origin servers; it is intended for use by intercepting proxies that are interposed as a means of controlling access to the network.
Responses with the 511 status code MUST NOT be stored by a cache.“ — ietf