Firewall/Lan modem

I have a 3com 56k lan modem connected to a windows 2000 pro workstation. The lan modem is shared by other windows 2000 workstations on our windows 2000 network. I will be using zonealarm for the firewall. Will I need to put the fire wall on the other workstations that will be using the internet or only on the workstation connected to the lan modem?