Ethernet and Internet security

If you intend to use Internet Connection Sharing on a home Ethernet network and you use an Ethernet card for your Internet connection, you must install a second Ethernet adapter in the computer that will be sharing its Internet connection. One Ethernet card stays connected to the Internet, and the other plugs into the Ethernet hub with your other computers. (It is not recommended that you plug your cable modem or DSL directly into the hub.)

Having two cards removes the risk that the contents of your computers could be visible to people outside of your home and be potentially vulnerable to malicious "hackers." Note that two Ethernet cards are not required if you have an internal DSL adapter or if you share a modem connection.