AC 900F can have 3 to 4 ETH ports. Each one of them is independent - there is no switch functionality (so each port requires different address in different subnet). ETH1 is normally used for Control Net (system network), but you can potentially use it in parallel with other protocols (like Modbus). ETH2 is dedicated for Redundancy Link, which is of course point to point connection. In case of single controller this can be used as other configurable ports (see below). ETH3 is used for Control Net Redundancy and as such most of what I wrote about ETH1 stands true for this port. If there is no Control Net Redundancy you can configure it freely. ETH4 (only for PM 902F) does not have a predefined function and as such can be freely configured for any required communication.
You can use up to 20 Ethernet Interfaces (communication objects) in any AC 900F, and even though each Ethernet port is independent, the sum of 20 interfaces is for the CPU as whole. These interfaces can run in parallel to Control Net and each other, but no other communication can be set up on Redundancy Link network.