PLANET CS-500 - Manual (Page 6)

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The IP addresses of WAN and DMZ are on the same subnet. This application is suitable if
you have a subnet of IP addresses from your ISP and you do not want to change any IP
configuration on the subnet.
4.2 NAT Mode Connecting Example
DMZ and WAN IP addresses are on the different subnet (as shown in the figure above).
This provides higher security level then transparent mode.