Problem when switching wireless networks!!!! please help

Hi,

I have IBM thinkpad, when I am switching from one wireless network to another network OR from LAN to wireless then I am able to connect to the new wireless network but I am not able to access the internet. When ever I get this problem I will use ipconfig/flushdns then ipconfig/release and then after repairing the connection I can connect to the new network and able to ac access the internet.
Why is this happening to me? Is this a common problem, Is there any solution or do I need to do all that procedure that I told?
Thanks in Advance.