Well you’re right.
As I said, I found out that all routers do not behave similarly.
At the time, I have 4 different routers : the one provided by my ISP, the WD N900, the Netgear WNDR4500 and an Apple AirPort Extreme.
I can configure the Netgear to share the same address (i.e. 192.168.0.XX) as the router provided by the ISP as long as I do not use the same addres range (i.e. : the ISP router goes from 1 to 50 and the Netgear goes from 51 to 100).
This is not possible on the WD. If I try to change it’s IP address to be on the same kind of Network (i.e. 192.168.0.XX) as the router provided by my ISP. If I do so, the WD N900 simply won’t change it’s IP address and will get itself a self choosen one, like 10.0.0.1.
I can also configure both the Netgear and the WD to get their IP address from the router provided by the ISP (i.e. 192.168.0.XX) and the to redistribute their own address over DHCP on their own IP address range (i.e. : 192.168.1.XX). This, I can’t do with the Apple Airport.
So, what I’ve done to be able to optimise the Gigabit Ethernet ports and USB ports, is that I let the WD N900 be the central one as it has 7 Gigabit Ethernet ports.
The Apple Airport Extreme is configured as bridged. It’s IP address is entered manually to be on par with the address range of the WD N900 even though it gets it’s internet connection over a switch.
Thus, I can access the USB port and the config page of the Apple Airport Extreme even though I’m plugged into my WD N900.
Now I will see if I can do the same with the Netgear, which would be interesting to take advantage of it’s 2 USB ports…