Blocking incomming IP addresses

Use a firewall (like ZoneAlarm)

The firewall protection also uses the port information to block certain types of incoming packets associated with common hacker attacks. For example, hackers use port scanner software to identify target computers for attack. Port scanners ?ping? ranges of IP addresses via port 7. If a computer responds to the ping, it becomes a target for further probing for open ports. By default, personal firewall software packages block these incoming pings on port 7 so that the computer does not respond.

as a cisco expert, i recommend that if u will not be accessing your network from the internet, u block all traffic from the internet except the the one you initialize internally.