Android Studio Error - android.os.NetworkOnMainThreadException

I have the script to make an URL request to server, like this :
DefaultHttpClient client = new DefaultHttpClient();
try {
} catch (IOException e) {
}

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but, when i click button 'Run App' and installed on my device, it says error on logcat :
FATAL EXCEPTION: main

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how to solving?
see this video for details :)