iptables is firewall used in linux servers. This is a command line tool. Most other firewalls like uwf, csf, apf etc actually use iptables to do the blocking, they are just programs make it easy to work with iptables.
ip table rules on CentOS 6
List Firewall rules
iptables -L -n
Flush Firewall rules
Block an IP
In this example, we block IP subnet 126.96.36.199/24.
iptables -I INPUT -s 188.8.131.52/24 -j DROP
Saving and Restoring iptable rules
To save current IP table rules, run
/sbin/iptables-save > /etc/iptables.rules
To restore it on boot, add following to /etc/rc.local
/sbin/iptables-restore < /etc/iptables.rules