iptables

#iptables -L

Displays ruleset of iptables

#iptables -I INPUT -p tcp -m tcp -s 192.168.15.254/26 --dport 22 -j ACCEPT

Accepts incoming SSH connections from IP range 192.168.15.254/26

#iptables -I INPUT -s "192.168.10.0/24" -j DROP

Drops all traffic from IP range 192.168.10.0/24

#iptables -A INPUT -p tcp --dport 25 -j DROP

Blocks all traffic to TCP port 25

#/etc/init.d/iptables save

Saves all IPtables rules and re-applies them after a reboot