ZyXEL and mixed NAT non-NAT LAN Subnets
The ZyXEL 1312 and 3925 range of routers don't allow the use of both public and private IPs at the same time on the LAN. There is an option in the web interface to have a 'Public LAN' subnet, but the problem is that the ZyXEL will still NAT all the traffic, and there is no way to configure the router to only NAT the private IP addresses and not to NAT the public IP addresses.
If you are wanting to use a mix of public and private IP addresses on your LAN, then the only option with the ZyXEL is to use 'Port Mapping' or 'Address Mapping' features. These are found in the NAT section of the web interface.
The Port Mapping tab will allow you to map ports sent to the Router's WAN IP to internal IP addresses, whilst the 'Address Mapping' will allow you to specify IP addresses in the public block allocated to you that will be used when a device makes an outgoing connection.
Example
Say AAISP have allocated you 81.187.x.0/29, and yu have a server on your LAN which you've given 192.168.0.100:
