Distinguish RADIUS requests from NAS device

Hi there. We have a network device using FreeRADIUS 1.x for authentication. This RADIUS server in turn queries an LDAP server. We wish to distinguish between "admin" and "login" requests but are struggling to differentiate between the two. At the moment we identify the device by its NAS address but cannot distinguish which type of login request is being made - so cannot then choose which LDAP groups to query. Our only solution so far is to run separate instances of RADIUS and direct the network device to each instance per the request type it receives. Can anyone this of another way to distinguish the requests ? Thanks very much.