Ping to iPad, Request timeout

I have some problem using my iPad2 Wi-Fi connection.

iPad is connected to wi-fi (iMac Ethenet->AirPort Sharing) and iPad's internet works fine - iPad's Safari displays google.com page well.

But if I send ping command to iPad from my iMac, It returns "request timeout" message. (My iPhone response to ping fine).

And I use iPhone applicatoin named "note2self" that provide wi-fi sharing.

On my iPhone, note2self display it's IP Address and I can download on my iMac by accessing to iPhone's IP Address.

However, "note2self" that installed iPad does not display it's IP Address.

I think there is some relation between Ping problem and note2self's wi-fi problem.

Is there somebody who know what's wrong with my iPad? ;_;

Help me Please.

(my iPhone version is 4.2.1, my iPad version is 4.3.3)