Q:

Send notification to all the devices connected to a Wi-Fi network

Is there any way to send a notification to the devices which are connected to a particular Wi-Fi network?
Say, I have a Wi-Fi network named "My Wi-Fi", which is not secured, that is any one can connect. A public network.
There may be N-number of users connected to "My Wi-Fi". These users can perform any kind of transaction, say online payment. 
Now if I want to turn off or shut down "My Wi-Fi" router or access point, these transactions may fail.
So before shutting down, I want to send a text notification to all the users connected to "My Wi-Fi" network. (User does not have any kind of app in their device, to push the notification.)
Is this possible?

A:

There is no standard method of sending (pushing) a message to all devices attached to a Wi-Fi network. If there was a way, it would be easy to find the specification and point to how it is to be done. Unfortunately, it is difficult to prove the absence of something.
As you clearly realized, it would be possible to do so if an appropriate page which you control was open in a browser, or application running, on their device. You could develop a framework where users have to sign on and keep a page open, or application running, in order to connect to your Wi-Fi.
Given that you control the router, it would be physically possible for you to write code which intercepted the packets being transmitted through the router and inserted such a warning within the HTTP of pages being sent to the various connected devices. This assumes that they are using HTTP to view normal pages.  You could, of course, also insert a warning in other protocols. Depending on your jurisdiction this might be illegal, or have other legal issues. I would consider doing so to be a Bad Idea™.

