ISPs typically have equipment available to them that makes traffic monitoring fairly easy. They typically don’t do it because in general, you and I just aren’t that interesting. Only when law enforcement comes calling with a warrant might they hook things up and start paying closer attention.
Most home routers simply don’t have the functionality to enable full-on monitoring. That’s not what they’re built for. Some might be able to expose what websites and other servers have been visited, but not the data exchanged as part of those visits.
However, access to the router – or rather the connections going into it – enable direct monitoring with additional, cheap equipment.
What physically has to happen is actually pretty simple. Someone inserts a hub into the connection between, say, the router and your computer. They then connect another computer to that same hub and run (free, open source) packet-sniffing software to monitor all of the traffic between your computer and the router.thanks