A system, method, and AAA (Authentication, Authorization, and Accounting) server in a packet data network. The AAA server authenticates users, authorizes services for the users when the users access the network, and generates a session identity that comprises a unique random value that is opaque, unpredictable, and not simultaneously re-usable. The session identity includes a session reference and an identifier of the AAA server, such as a realm identifier assigned to the AAA server, that is usable to route queries containing the session identity to the appropriate AAA server. The queries may be routed to the appropriate AAA server by other AAA servers configured with routing tables that match realm identifiers to AAA servers. Alternatively, a specialized AAA server is configured with a routing table and routes the queries to the appropriate AAA server.