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.
Let configures your router to do NAT gateway so that all traffic sent to eth1 interface will be modified by replacing the source IP of each datagram by public IP (80.0.0.2): 
If we return to make the machines ping the Internet from our local network, we see that now come without problems: 
If we check IPTables counters, see how effectively the rule that we created is being used properly: 
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!
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