When configuring NAT, the Internet interface is considered to be what?

Defining NAT Inside and Outside Interfaces
The first step to deploy NAT is to define NAT inside and outside interfaces. You may find it easiest to define your internal network as inside, and the external network as outside. However, the terms internal and external are subject to arbitration as well. This figure shows an example of this.

Example: Allowing Internal Users to Access the Internet
You may want to allow internal users to access the internet, but you may not have enough valid addresses to accommodate everyone. If all communication with devices in the internet originate from the internal devices, you need a single valid address or a pool of valid addresses.

This figure shows a simple network diagram with the router interfaces defined as inside and outside