distinction between users on different AP (talking to the same radius server)

Hey everyone, I was just wondering for your opinion on this issue- I want to spread several access points in different locations (they all talk to a central radius) and then i want to distinct one location from another for example user foo can login from either location but id like to make the distinction from which ap he got connected from... whats the best way to do that? I was thinking of one method which is to configure in each AP a different subnet mask for the DHCP allocations and then make the distinction based on that but I'm looking for a more elegant way. Thanks guys, Liran.