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More concretely... the traffic will go out of the computer? Will the NIC send out the request for the 2nd IP to the LAN and will answer itself, or will the traffic somehow stay inside the computer?

OS: Windows (2003, 2008)

Note:
I'm focusing on Windows, but the question is mostly theoretical, as I was trying to go into the fine details of how requests are routed in different scenarios (one of which includes having both the source and the destination IP on the same box). :)

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3 Answers 3

up vote 3 down vote accepted

It depends on the network stack of the operating system.

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Sorry, could you detail a little bit more? :) Consider we only have 1 NIC with 2 IP connected to a LAN. –  Ali Oct 9 '10 at 15:18
    
Most modern Operating Systems will realize that the traffic should not leave the computer and the packets will not travel the network. Older OS (like very old) will send it over the LAN anyway. –  adamo Oct 9 '10 at 15:25
2  
he means that different operating systems will behave in different ways. You need to give a bit more detail, Ali, if you want a good answer. –  RobM Oct 9 '10 at 15:25

All local address are mapped to the loopback interface for routing on all major OSes. That means the traffic will stay "inside" the computer and will never hit the wire.

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As others have said, the results will vary according to your operating system. On most modern OSs you'll find that it never reaches the wire, but if you can specify which operating systems you are referring to somebody will be able to provide a more definitive answer.

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I'm focusing on Windows, but the question was mostly theoretical, as I was trying to go into the fine details of how requests are routed in different scenarios (one of which includes having both the source and the destination IP on the same box). Thanks for the answer anyway! :) –  Ali Oct 9 '10 at 16:08

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