Some cable ISPs allow you to use your Ethernet card in a home-based LAN. The Ethernet card works in combination with your cable modem to connect to your ISP. However, the ISP might restrict the number of PCs you can attach to that LAN because of the capability it gives you to share the Internet connection across several PCs.
Before you set up a home Ethernet LAN, make sure you check into the agreement you have with your cable ISP. Because it operates on an Ethernet connection to your home, it may be able to detect how many systems you have attached, and may shut off your service if it finds you breaking the rules.