The basics.

Client uses SCAN hostname. This resolves to an IP address (or addresses). The client uses the IP to connect to the SCAN listener.

The SCAN listener (via the remote_listener setting) is continually informed by the various database and ASM instances of what services are available, and on which hosts.

The SCAN listener processes the client's request and sends a redirect back to a specific host with a local listener supporting the specific service.

The client receives the redirect. It resolves that hostname (typically a VIP hostname) to an IP address (typically a VIP). It now connects to that IP and local listener on it, requesting a connection to the required service.

FROM:

Difference between SCAN and VIP in RAC ??

https://community.oracle.com/message/11242902