When a wireless client associates and authenticates to a WLC, it places an entry for that client in its client database. This entry includes the MAC and IP addresses of the client, security context and associations, quality of service (QoS) contexts, the WLAN, and the associated LAP. When a client roams to another LAP associated to the same WLC, it just updates the client database with the new LAP information so that the data can be forwarded appropriately to the client. When a client roams to a LAP associated with a different WLC, either in same or different subnets, it sends the information in the client database to the new WLC. This helps client to retain its IP address across roams and maintain uninterrupted TCP sessions. For more information on roaming in the WLC environment.
For the further description following URL for the WLC may help you
http://www.cisco.com/en/US/docs/wireless/controller/5.1/configuration/guide/c51ovrv.html