Connection limitation by device on website?

Working on a web project, one of our customer would like to add a limitation feature on member connection: he want to limit number of simultaneous connection for same member on a website, using different devices (eg. smartphone, tablet, PC).

I am wondering on how to develop this feature, and which data to use to know if the same member is connecting using different devices. Indeed, I want to this "properly", and not to impact a customer who is not doing multiple connections.

What is the best way to proceed? As we are in standard HTTP request (asynchronous), we don't know if a member is "connected" (in fact there is not a real connection). What data should I use to detect a different device? Date of last request, IP, browser version?

Some very known websites are using this feature, for example Netflix.

Thanks for any advice or howto on this subject :)