This fits in nicely with another security Topic
on my web browser how do I spoof my location
Cable Modems, routers, and maybe some other devices have NAT firewalls in them. Here is a good tutorial on how NAT firewalls work.
The last section in Part one that parents with children at home might be interested in:
Protecting one “high-value” machine from the rest of the network:
This uses two NAT routers to protect a high value machine (your computer) from the rest of the LAN (your children’s devices).