Use this command to add an ip address to be blocked by iptables:
<code> iptables -I INPUT -s x.x.x.x -j DROP iptables -I INPUT -s x.x.x.x -j LOG </code>
This example inserts a rule at the top of the table to block all incoming requests from ip x.x.x.x, and then inserts a second rule at the top to first log any connection attempts from this address (before they are blocked).
To see what rules are currently configured, use:
For more info, see this entry in the O’Reily Linux Hacks book.