There is no facility to specify IP addresses for anything within OneCare. Furthermore, there is no "hub based" antivirus system. If you have multiple PCs running OneCare activated with the LiveID for a single subscription, they each have their own antivirus engine, database, and scheduler. One or more PCs can be made into a Hub PC for the Circle, meaning that the hub will display the status of OneCare on the other PCs.
-steve