rosjava_android WIFI set up in a windows based virtual box

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I have modified the the android_tutorial_pubsub package to let it connect a roscore on PC and write a talker on PC to send message to the android phone. It works OK, but when I move to virtual box in windows 7, I hava no idea how to set up the ip for it to work. Does anybody know how to do this? Thanks. BTW, I found that the connection between the android phone and the PC can only be established when they are connected to the internet. I have tried to connect the PC and the Phone to the same router without connecting the router to the internet, the connection is not established. It only works when I connect the router to the internet. Is there a method to let it work without the internet? Thanks.