If another machine was on network 2 and it wanted to ping a machine on network 1. How would it know where to send the packet as soon as it realises its not on the local network? Itd go to the default gateway right? that means the default gateway(slackware linux machine) would have to have a route for the 192.168.0.x/27 subnet right? otherwise the packet will get lost in transit??
To get a packet from network 2 to network 1 there has to be a machine connected to both networks that will route pachets between them. In this case (the third diagram on the page you mentioned), this is the Netware 6 server in the bottom right hand corner. The Linux firewall is also acting as a router, but between network 2 and the internet.
A workstation on network 2 would need to know about the Netware 6 router in order to successfully ping the PC on network 1. There are a few ways of doing this. Firstly, you could set the default gateway of the workstation to be the address of the Netware 6 server. This would work but how would the workstation then get internet access?
Second, you could leave the workstation's default gateway pointing to the linux firewall but add a route on the workstation to tell it that the gateway for the network 1 address range is the Netware 6 server. That would work but involves manual configuration so might be awkward in a large network with many machines
Third, you could leave the workstation's default gateway pointing to the linux firewall but add a route on the linux firewall that redirected traffic for network 1 back out of the same firewall interface and off to the Netware 6 server. This also could work but generates extra network traffic due to the double hop and would also make the firewall difficult to configure
Hope that helps