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Segment: A section of a physical network (wire). Segments are delineated by routers and similar devices that selectively pass or ignore packets based on a defined criteria (e.g. subnet or MAC address).
Subnet: A logical network defined by performing a bitwise logical AND on a given IP and the corresponding subnet mask. More than one subnet can exist on a given segment.
In a multihomed machine that has adapters attached to different segments, each adapter should (in almost all cases) appear on different subnets. If both adapters appear on the same IP subnet, when the machine needs to route a packet to another machine on the same subnet, which adapter should it use? As far as it can tell, both adapters are attached to the same subnet. Therefore, it doesn't matter which adapter is used. So it might or might not use the correct one.
Network segments (that need to be able to communicate with one another using TCP/IP) separated by routers should appear on different IP subnets. If two segments appear on the same IP subnet, when a machine on one segment needs to route a packet to another machine on the same subnet, it will simply place the packet on the local segment (since it is attached directly to the destination subnet, why should it attempt to use a router). If the destination machine is actually located on the remote segment, it will not receive the packet.