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By putting guest traffic in a different subnet, guests will not be able to communicate with devices on your network.

You should then configure the second router to have an open Wi-Fi network and give IP addresses on a different subnet.

There are many resources available on networking that explain home subnets in depth, but simply put, a subnet is a set of addresses that a device can communicate with, without any help from a router.

The ETH Zurich team scoured a massive sample of internet traffic between 2004 and 2012, finding more than 19 million unique active subnet addresses.

The two of you are probably connecting to Facebook from the same IP subnet, or else using Facebook on your cellphones from near-overlapping geocodes.