Network Authentication Required

The client needs to authenticate to gain network access, common on captive portals.

511 Network Authentication Required means the network you're on, not the site you asked for, wants a login first. It's the captive-portal code: airport and hotel Wi-Fi intercepts your first request and answers 511 with a login page. The code exists so portals don't poison caches by answering 200 with the wrong content.

What 511 looks like on the wire

GET http://example.com/ HTTP/1.1
Host: example.com

HTTP/1.1 511 Network Authentication Required
Content-Type: text/html

<meta http-equiv="refresh" content="0; url=https://wifi.example.net/login">

About 5xx Server Error

The server failed to fulfill a valid request. The problem is on the server side, not the client.

How to fix 511 Network Authentication Required

Open a browser and complete the network's login page.
If the portal doesn't appear, visit a plain http:// site to trigger the redirect.
After logging in, retry the original request - it never reached its destination.

Where it's defined

511 Network Authentication Required is defined in RFC 6585 §6. MDN's 511 reference covers browser behavior and examples.