A 511 Network Authentication Required error is a webserver’s code for: "You need to agree to terms or log in to the Wi-Fi network before you can browse the web.".

How do I fix or troubleshoot a 511 Error?

Common steps include reloading the page in your browser, checking for typos in the URL, verifying server status, clearing browser cache or cookies, and contacting the website owner or server administrator if the error persists.

511 Error | Error-Message.Net

A 511 Network Authentication Required error indicates that the client needs to authenticate to gain network access.

This code is specifically designed for “captive portals”—the login and agreement screens encountered when connecting to public Wi-Fi networks at airports, hotels, coffee shops, or university campuses.

Determining why

Before you agree to the terms of service or pay for access, the Wi-Fi router intercepts your web requests. Instead of secretly redirecting you and breaking SSL certificates, modern routers send a 511 status to tell your operating system: “A captive portal is blocking access”.

Steps you could possibly take:

Open your browser’s captive portal prompt - Open a fresh browser window and navigate to an unencrypted address like http://neverssl.com or http://example.com to trigger the network’s sign-in screen.
Accept terms or enter credentials - Complete the required agreement or room number verification.
Turn Wi-Fi off and back on - Reconnecting prompts modern operating systems (iOS, Android, macOS, Windows) to pop up the captive network login assistant.

Source: RFC 6585 Section 6 (Additional HTTP Status Codes)