Who is connected to network

Hi All, Is there a way to see who is connected to your network. Say if you have a wireless network and you need to know if someone got it right to get onto your network. How do you monitor that and how do you prevent it? Even on a normal network how could you monitor who is connected to your network? Regards _____________________________________________________________________ For super low premiums, click here http://www.dialdirect.co.za/quote