According to text from an ADSL gateway Netgear make
ftp://downloads.netgear.com/files/DG...UM_14Oct10.pdf
NAT automatically assigns private IP addresses (10.1.1.x) to LAN-connected devices.
• Enable. Usually NAT is enabled.
• Disable. This disables NAT, but leaves the firewall active. Disable NAT only if you are sure that you do not require it. When NAT is disabled, only standard routing is performed by this router. Classical routing lets you directly manage the IP addresses that the N300 wireless modem router uses. Classical routing should be selected only by experienced users.
That doesn't sound like a modem only mode, as to me it reads as if the gateway still retains the external IP, and you would still have to use private IP addresses on the devices connected to the Gateway
I would assume the Cable version works in the same way