Hello, I have a client PC in the 192.168.3.0/24 network behind a Fortigate firewall which performs NAT.
This firewall is likewise connected on the 192.168.4.0/24 network to my ADSL router which also performs NAT.
So I have double NAT going on from my client PC, out to the internet.

My client PC can ping it's default gateway on the 192.168.3.0 network (Fortigate firewall interface 192.168.3.254). My client can also ping to the internal interface of the ADSL router (192.168.4.254). But my client cannot ping out to the internet.

It would seem that double NAT is causing this. My question is how does double NAT prevent the pings from the client to the internet working ?

Thank you for any help in me understanding this.