I finally managed after a lot of pain to properly connect arduino and his ethernet shield to my laptop. My laptop is connected to wifi, while the router is wired to my other pc in another room.
I managed, with ICS (Internet Connection Sharing) to connect Arduino to the laptop and now i can host stuff on the specified id.
But if i try to access to that ip from another pc in the house, nothing happens, because the communication apparently is just between my laptop and my arduino.
How can i make the IP of my arduino visible to anyone connected to my home's router?