511 Network Authentication Required
The airport wi-fi captive portal, with a status code of its own.
RFC 6585 created 511 for networks that demand a login before releasing the internet — airport, hotel and café wi-fi. The one returning it is not the requested site: it is the network's equipment, intercepting, with a link to the authentication portal in the body.
The reason is cache protection: before it, portals answered 200 with the login screen, and browsers stored the screen as if it were the site. With 511, the protocol itself declares that this is not the requested page — it is the toll booth.
HTTP/1.1 511 Network Authentication Required
Content-Type: text/html
<meta http-equiv="refresh"
content="0; url=https://portal.wifi.exemplo/login">