This may be a dumb question but I really don't get it.
Let's assume we have two networks. Network A and B. They have the following devices:
- Network A
- Device X (IP: 192.168.0.10; subnet mask: 255.255.255.0)
- Network B
- Device Y (IP: 192.168.1.11; subnet mask: 255.255.0.0)
- Device Z (IP: 192.168.0.10; subnet mask: 255.255.0.0)
- Router that connects A and B
This works out when Device X would send something to Device Y because X would notice that Y is not in his network and would send his message to the gateway. But when Y wants to answer X's message, Y will think that X is in his network because of his subnet mask (255.255.0.0). That would not only be an error but also a security problem because now Z would receive the answer that was intended for X.
Edit: Is this correct? Is this a serious security flaw?