Specifying Inside and Outside Interfaces

You must mark interfaces that participate in NAT translation as residing on the inside or the outside network.

Caution: Only packets routed between an inside and an outside interface are subject to translation.

You can unmark an interface by using the no version of this command.

ip nat

Use to mark an IP interface as participating in NAT translation.
Use the keyword (inside or outside) to specify the side of the network on which the interface resides.
Example
host (config-if) # ip nat inside
Use the no version to unmark the interface (the default) so that it does not participate in NAT translation.
See ip nat.