In previous related question, one key point was to disable DHCP to prevent IP address conflicts in my old unused router. I was also advised to change its IP address to be an IP outside the DHCP range of the Trendnet gateway router.
Before getting this advice, I just installed the router to see what would happen. In Figure 2 (in previous question), both XP2 & PS3 actually had internet connection through this old router and the Trendnet Router. Yet, its IP address was 192.168.1.1, whereas the Trendnet IP address was 192.168.0.1.
(But since DHCP was not disabled, there were some brief intermittent internet outages on other units; and then finally unit W7 had no connectivity until I turned off the power of the old Router.)
Why was it necessary to change the old Router's IP address to 192.168.0.y? In general, my understanding is that simple switches do not have any IP address associated with them. I ask this question because, as I said, just plugging in the old router seemed to work OK as a switch (except for the mess it caused due to not disabling DHCP).