Hello!
been a while since I talked about NetGUI , free network simulator in the style of Packet Tracer, but machines designed to work with GNU / Linux. Today we will see how we can use, for example, illustrate the operation of NAT and its configuration on a Debian machine.
The scenario with which we work is as follows:

We have a local network with two computers (PC1 and PC2) on the 192.168.1.0/24 network connected to the Internet through the gateway router. This router is connected to the ISP's router (network 80.0.0.0) and the Internet is represented by 2 web server host (network 90.0.0.0).
The following images show how they have configured the network interfaces and routing tables for each machine:




If you now try to ping from local network to any Internet machine, it is clear that we will not do, because We have a private address and our gateway is not doing NAT. The machines outside the local network will not know how to answer our requests for echo.




NetGUI is a very interesting tool that allows you to create complex scenarios to test different configurations, so I recommend you to take a look.
Greetings!