Implment RFC1918 or Deny Private IP in Internet.

ACL to Deny Private IP:

Internet not allows Private IP Address. So ISP should be denied Private IP Address in internet.

Create Access List to filter Private IP address.

Router(config)#Access-list 100 deny ip 10.0.0.0 0.255.255.255 any             // Deny Class A private IP

 Router(config)#Access-list 100 deny ip 172.16.0.0 0.15.255.255 any           // Deny Class B private IP

Router(config)#Access-list 100 deny ip 192.168.0.0 0.0.255.255 any           // Deny Class C private IP

Router(config)#Access-list 100 deny ip 127.0.0.0 0.255.255.255 any           // Deny loopback IP Address

Router(config)#Access-list 100 deny ip 169.254.0.0 0.0.255.255 any           // Deny APIPA

Router(config)#Access-list 100 deny ip 224.0.0.0 15.255.255.255 any         // Deny Class D Multicast (Private Multicast range 224.0.0.0/4)

Router(config)#Access-list 100 deny ip 240.0.0.0 7.255.255.255 any           // Deny Class E Multicast

Router(config)#Access-list 100 deny ip 255.255.255.255 0.0.0.0 any           //Deny Broadcast

Router(config)#Access-list 100 permit  ip any any

Router(Config)#int fa0/0

Router(Config-if)#ip access-group 100 in