Most of the times I have noticed, whenever someone configures a NAT in a Cisco PIX firewall 525 or a Cisco ASA firewall, he says my NATing is not working.  This is true especially if you are configuring a NAT for a real IP (which is in use) to private IP. The moment NAT is configured in a firewall he assigns the private IP to his machine and check whether the NAT is working?  Usually it fails to work. The reason behind is the arp cache is not cleared in the firewall, switch and the router.

Next time whenever you configure a NAT for a used real IP make sure you clear the arp by “clear arp-cache” in your Cisco router and Cisco Switch. “Clear arp” in your Cisco firewall . To do so simply use the below command from the privilege mode in your Cisco device

ITKE-ROUTER#clear arp-cache

ITKE-SWITCH#clear arp-cache

ITKE-FIREWALL#clear arp