When I connect to a hotel network that does not require a WPA key, on my PC, the first browser access will be trapped and directed to the login page. This is understandable, and I can guess the network equipment provisions access by MAC address.
On my Android (5.0) phone, a Sign-in to Network screen opens and it shows that same web page as what I see on the PC. What is the protocol that makes this happen?
Best Answer
Direct quote from How Automatic Detection of Captive Portal works (follow link for full text)
Basic strategy behind Captive Portal detection
The Automatic Detection of Captive Portal mechanism is based on a simple verification, done by the Operational System (OS) of the client device (smartphone, tablet, laptop).
It simply tries to reach a specific URL and verify that such URL returns a well-known result.
Further Readings
Windows 7 Network Awareness: How Windows knows it has an internet connection
Captive portals - MSDN