Clients must always refer to the database by using the network resource, not the physical hostname. The network resource is an IP address that can move between physical nodes during failover. The physical hostname is a machine name.
For example, in the tnsnames.ora file, you must specify the network resource as the host on which the database instance is running. The network resource is a logical hostname or a shared address. See How to Set Up Oracle Database Permissions.
Oracle client-server connections cannot survive a Sun Cluster HA for Oracle switchover. The client application must be prepared to handle disconnection and reconnection or recovery as appropriate. A transaction monitor might simplify the application. Further, Sun Cluster HA for Oracle node recovery time is application dependent.