2Wire modem giving me wrong IP address

Hi -

I don't know what I did to cause this, but one of the systems on my home network is no longer getting the 192.168.1.NNN IP address. Instead, I'm getting assigned my public IP address, which causes problems getting to this system from other machines on the network.

Any ideas? I've rebooted with no change in results. Thanks for any ideas.

EDIT: the system in question is a Mac, running 10.6. It's worked with 2Wire for ages.