Cisco supports the diagram on the left as it relates to the Cisco Small Business WRVS4400N Router. As in, both sides of the WRVS4400N are 192.168.1.*, and the routing is handled by the next hop (the NAT router).
The NAT router gives out 192.168.1.* IP addresses (statically or DHCP does not matter to Cisco)
Hypothetically, the NAT router's WAN IP address is 184.108.40.206 with a subnet mask of 255.255.255.0
But the diagram on the right is not supported by Cisco. I am not getting straight answers about why. Basically, if both sides of the WRVS4400N are 8.105.128.*, and the next hop is the ISP Gateway, all of a sudden this is "a limitation of the device". They can't seem to explain it very well, but they are pointing me to Cisco Enterprise which is (from what I have seen) more difficult to configure and more expensive.
The only technical explanation I can think of that actually might make sense is that 192.* is Class C and 8.* is Class A. The router is said to only support Class C. Something about the difference between Classful and Classless Routing.
Do you think they just want me to purchase a more expensive product or do you think there might really be a difference?
This does not really relate to our purchasing decision. I think we will be using Enterprise just so Cisco will support it. I ask this question because I want to understand what is going on. If there really is something there, I would like to take this opportunity to expand my understanding.