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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.

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That is actually somewhat misleading. On a given segment, every frame may be sent to every house. This really limits the issue to a few dozen houses at the most. (The cable modem really knows nothing about IP packets anyway; it listens for layer-2 frames destined for it, not layer-3 packets.)

The cable provider takes steps to lock you out of the cable modem. That doesn't mean that someone can't plug in an altered device to see traffic, but the reward would be very small. Also, the growing use of encryption for anything that matters really means that this is a lot of work for almost no reward.

There really is almost no point to what you are asking about.

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