Why RAC need a VIP (Virtual IP address)
Importance of VIP
Real Application Clusters in 10g, however, don’t particularly want you to connect to physical IP address associated with network interface.
Doing so means IP packets are routed to a physical MAC address, so that if that address ever ceases to exist (such as when a server dies), we have to wait for TCP/IP networking protocol itself to work out that packets are undeliverable.
That can take up to 10 minutes, and would mean failover in a RAC can potentially be very slow.
Instead, Oracle wants users to connect to a Virtual IP Address (VIP). That’s an IP address that’s bound to a software-controlled MAC address and since it’s software controlled, the software can arrange for failures to be handled a lot quicker than plain old TCP/IP stack (in seconds, usually).
The VIP for a RAC node is quite often the normal, real IP address plus one so, in my case, that would imply a VIP of 192.168.1.111. I won’t be needing this until it comes time to installing the Oracle software, but it’s good to plan ahead.
Even if check fails you can continue the installation,by configuring vipca seaparately , even if that IP does not exist (no need that you own that ip).