Also, if you're on a LAN, you might need to unselect the "Internal Modem" option. OSX has been known to prefer that setting.
After I install osx on any system, I delete the "internal modem" option and force the machine to keep the "Built-In-Ethernet" option. Of course, don't do this if you are using a dial-up connection to the interent. (Since you didn't mention that I'm just assuming the possibilty that your on a LAN).