Why does my app crash when I try to display my ip usig Inet?

android don't give u permission to run any network related work direct in main thread so you get this Exception :

android.os.NetworkOnMainThreadException 

run your code like this:

Thread thread = new Thread(){
        public void run(){
            try {
                Inet4Address ip= (Inet4Address) Inet4Address.getLocalHost();
                String s=ip.toString();

             TextView text= (TextView)findViewById(R.id.txt);
              text.setText(s);
                Log.e("my ip=> ",s+" <<");
            } catch (UnknownHostException e) {
                e.printStackTrace();
            }
        }
    };

    thread.start();

it's just run you code another thread...