Setting Different Subnet in Portal

Guys,

is it possible to set a device to a different subnet (from the controller) in the device configuration of Portal? Im setting my controller slot 2 to a subnet of 255.255.252.0 and what to set a device in it's same tree to a subnet of 255.255.254.0 . Currently it will not allow me to change the device's subnet to anything (its grayed out) and is set to the same as the Controller Slot 2. Attached are some images to kind of show you what I mean. We are trying to change these so that only certain devices talk to others.

The subnet mask in the IO Controller and in the IO Device must be the same.
Why do you want to have different subnet masks in the first place ?
Change the subnet mask in the CPU, and the IO Devices will be updated accordingly.

The reason why we want different Subnets is because we have some devices on the overall network that we dont want to be able to talk to anything BUT the controller. We are trying to create a Bit Mask so that some IO devices can NOT be accessed by other devices on the same controller....if that makes any sense

Which IO devices need to be isolated from the others? Most IO devices are "dumb" and wouldn't be able to affect the others.

Ultimately, I don't believe Portal will let you change the subnet mask of your IO devices. If you want network isolation, I'd strongly recommend using a CM, like you already are for some of your devices.