Oracle 10g Cluster Ready Services enables databases to use a Virtual IP address to configure the listener on. This feature is to assure that oracle clients quickly failover when a node fails.
When a client connects to a tns-alias, it uses a TCP connection to an IP address, defined in the tnsnames.ora file. When using RAC, you define multiple addresses in your tns-alias, to be able to fail over when an IP address, listener or instance is unavailable. However, TCP timeouts can differ from platform to platform or implementation to implementation. This makes it difficult to predict the failover time. In theory, the failover time can add up to two minutes (default TCP timeout). In Oracle 10g, when a node fails, the Virtual IP address is taken over by (one of) the remaining RAC node(s).