How To enable NAT/PAT on your Cisco IOS router 

To enable NAT on your IOS powered Cisco router, follow these simple steps.

1.) Identify your outside (public) interface. This interface is usually a serial
    interface, dialer interface, or another ethernet interface.

2.) Identify your inside (private) interface. This is usually ethernet0/0 or
    fastethernet0/0.

3.) Specify what IP netblock you want to NAT or NAT all subnets.

4.) Enter the configuration and test.

Example 

In this example, we use serial0/0 for the "outside" interface and ethernet0/0
for the "inside" interface. We also NAT every IP subnet from the "inside"
interface:

	router(config)#
	! goto your "outside" interface and tell it how to NAT
	interface serial0/0
	ip nat outside
	exit
	! goto your "inside" interface and tell it how to NAT
	interface ethernet0/0
	ip nat inside
	exit
	! build an Access-List to tell the router what subnets to block.
	access-list 101 permit ip any any
	! tell the router to use the new Access-List and to NAT using PAT
	ip nat inside source list 101 interface Serial0/0 overload
	exit
	copy running-config startup-config

Now you are setup to access the Internet with your internal subnets. 

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