java.net.UnknownHostException:Unable to resolve host "graph.facebook.com" when i run android facebook app after closing emulator

I can able to see facebook groups when I run app for the first time. But when I close the emulator and run my application again, I am getting following response.

{Response: responseCode: unknown, graphObject: null, error: {HttpStatus: -1, errorCode: -1, errorType: null, errorMessage: java.net.UnknownHostException: Unable to resolve host "graph.facebook.com": No address associated with hostname}, isFromCache:false}

I used following code for fetching facebook groups

    Request request = new Request(session, "/me/groups", null, HttpMethod.GET,
            new Request.Callback() {
                @Override
                public void onCompleted(Response response) {
                     //code for displaying groups in list view as response contains groups information

            });
    request.executeAsync();

Please help me in resolving this issue.Am I missing any session handling usecases ?