If you want to block a specific IP address from a server (say, they are attacking you):

iptables -I INPUT -s address -j DROP

To block an entire subnet:

iptables -I INPUT -s ipaddress/range -j DROP

Example, using 192.168.1.5

iptables -I INPUT -s 192.168.1.5 -j DROP

or, the entire 192.168.1.0/24 range

iptables -I INPUT -s 192.168.1.0/255.255.255.0 -j DROP

or

iptables -I INPUT -s 192.168.1.0/24 -j DROP