How does a firewall let several computers share one internet connection?
The firewall shows up as a single address on the internet. The internet service provider (ISP) whom you pay for internet service (dial-up, DSL, ISDN, cable, or leased) takes the packets (see prior question) and passes them on to the internet. The ISP also identifies packets intended for your firewall and passes them on to you. All packets for your network are addressed (see prior question) to the firewall. The firewall sorts out which of your computers is to receive each packet. Since the computers behind the firewall (on your network) are not visible (see prior question) to the internet, the ISP does not know whether you have one or many.
