How was Netflix able to detect if you're connecting to the same household wifi?!

I was talking to friend about a new product that we've been building and mentioned the Netflix use case from my previous post He was asking me how exactly would we prevent fraudulent logins or prevent promo abuse with our new tool This led me to explain the basics of our new tool to him. The whole point of device/browser fingerprinting is that : The specific combination of non-unique attributes can be used as a unique identifier. You have the latest iOS? so does multiple people. You use the Safari browser? so does many other people. You are in this particular time zone? so are other people. You have this particular iPhone with this particular hardware inside? So does other people. But if you put that and a bunch of other information together, as in The Particular User Agent + Specific time zone + Particular phone model + Location (via IP) + Geo location (based on permissions) + Specific hardware + Specific software + Bunch of other info, then it really narrows down the possibilities and eventually, they can pinpoint a single user.