The big difference between real and virtual IP addresses is that the real IP address is logically bound to a specific piece of hardware (MAC address on the network card); whereas the VIP address is controlled by software and its association with a particular piece of hardware can be changed dynamically.
If a database machine were to fail when a client program was attached to it through the real IP, the client program would wait a long time (tens to hundreds of seconds) for a response from the server before deciding that the server machine had failed. Because Oracle is using VIPs, when a machine fails, one of the other instance will detect the failure very rapidly and take on the virtual IP of the failed machine to handle the current requests. This reconfigures the system to stop future connections going to a failed VIP (until the machine and instance are live again). This means that clients will not experience a long wait if the instance they are addressing fails.
SCAN (Single Client Access Name): Scan is the another aid to minimize waits caused by network problems. Oracle 11.2 introduces SCAN as part of Oracle Grid Infrastructure.