I've been reading Computer Networking: A top down approach and I don't understand something about cable internet. The book says that since cable internet is a shared medium, every packet sent by the head travels to every home connected to the downstream. I know that the cable modem won't be listening for packets that isn't addressed to its IP, but couldn't someone alter that so it collects all the packets that are sent on the downstream, since it will pass through it anyway?
One important characteristic of cable Internet access is that it is a shared broadcast medium. In particular, every packet sent by the head end travels downstream on every link to every home and every packet sent by a home travels on the upstream channel to the head end.