If the router has a built-in firewall, then it will definitely protect the computers downstream from the other computer, though they may be able to request data from that computer.
I have a hardware firewall built from an old P4 computer and running Smoothwall Express
, which is a free firewall OS. It allows for a RED+GREEN+ORANGE configuration where the RED LAN port is the internet, the GREEN port goes to your protected LAN and the ORANGE port handles an internet-accessible server. The GREEN LAN is fully protected and can't access the ORANGE and vice versa without being specifically allowed by the administrator.