On firewall machine:
1) Put router IP in /etc/defaultrouter and /etc/hostname.le0 (eithernet port name)
2) Put beginning IP (of range assigned) in /etc/hostname.le1 (eithernet port name)
3) Edit /etc/netmasks and duplicate in /etc/inet/netmasks
On all systems
1) Edit /etc/hosts for new IP's
2) Boot system