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Are you attempting to get a new IP address from your cable provider? Because you will need to wait for the DHCP lease to expire.
 
Well on my PC I can just type in command prompt: "ipconfig /release"

And in Tiger I could change the network setting from DHCP to manual (and assign a IP of 0.0.0.0, thus releasing). However in Leopard this does not seem to be possible.
 
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