It troubled me that a Modbus device would only allow a 1:1 connection on the port. This flies directly in the face of the whole point to ethernet communications.
So I ran a test -- I took a Modbus/TCP server I had been working with (Not a S7-317-2PN/DP -- I'll try that tomorrow morning) and fired up Modbus TCP software from two separate clients.
Both communicated simultaneously.
My experience in both writing and debugging TCP communications is the port is opened and closed. (I found this the hard way in a set of touchscreens that had severe latency problems in communicating with the controller)
I haven't "sniffed" this session (both are communicating as I write), but I suspect that is what is happening. The other possibility would be that the port opening is done in "promiscuous" mode, where it responds to TCP requests from different IP addresses.
The server device I'm using doesn't support Modbus over UDP, but some devices do support it, where opening and closing the port is unnecessary.
Mind you, this doesn't necessarily apply to the S7 communications, I'll follow up tomorrow with that.