How Database Connections are Created When Using SCANs
Figure Load Balancing Actions for Oracle RAC Connections That Use SCAN
1. Public IP- Public IP is used for Direct connection to a particular instance only.
2. VIP- it's used for transferring the connection to another VIP in case of any failure.
3. Scan IP- its a new concept from 11g R2 onwards, wherein it has pre-information of which node has how much of load and it balances the connection between multiple instances.
Scan IP max cab be 3 (and Oracle has tested it and as per there internal result, 3 is more than enough for N instances.)
Remote_listener will have your SCAN_IP.
The local listener will have your VIP of a particular node.
VIP will do failover for your existing connection and incoming new request for that IP
SCAN IP will have re-information about load and will redirect your request to respective less load VIP
if you avoid VIP then you are left with public IP i.e your network card assigned to your server. That public IP is not able to failover in case of any failure.
Private IP, it's only for internal communication of RAC components maintained and managed by Oracle software itself