I have some systems running on Ubuntu in the private IP range 192.168.2.0-255 . These systems are connected to a switch and the switch is connected to the ISP's modem. Neither the switch nor the modem support firewall options. I don't have any firewall device and I'm not willing to individually configure firewalls on all the systems (via gui/iptables). Is it possible to make an Ubuntu system into something like a software firewall, so that all the traffic/packets sent to or from the WAN(internet) would be allowed/denied based on its firewall rules?