pls help: restrict authentication for certain realms from particular clients

Hi all, We just upgraded our freeradius server from 1.7 to 2.1.1. I'd like to restrict authentication requests as following: client A accepts: @realm1, @realm2 client B accepts: @realm2, @realm3 so that requests like follwoing will be rejected: @realm3 to client A @realm1 to client B Would anyone please help? Sorry for newbie question. Thank you very much. /ST Wong