In the time when 802.11g was new and exciting, Coyote lived in a house with some others and a friendly ghost. The ghost was always messing up the computers, until Coyote asked him not to. They lived together in a small town with a wireless ISP.
Coyote was working on a Windows computer belonging to one of the others and noticed he could see Windows shares of the ISP's other customers. “This is not good,” said Coyote. He realized that all of the ISP's customers were effectively on the same LAN. He knew this could put the others in the house who's gifts were not the same as his in danger.
So Coyote took an old computer and installed linux on it. He put two Network Interface Controllers into the PCI slots and he connected the wireless one to the ISP's network. Then he connected the other to his router in the house.
echo “1” > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
The magic was simple, but powerful. The house was protected. Coyote was done.