You can place multiple address entries for a single connection alias in the TNSNAMES.ORA file. This means that you can connect to a database, even if some kind of physical failover occurred. Look at the following example:

ORA1 = (DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS =
(PROTOCOL = TCP)
(HOST = Host1))
(ADDRESS =
(PROTOCOL = TCP)
(HOST = Host2)))
(CONNECT_DATA=((SID=ORA1))))

Suppose Host1 is down, then every new NET8 connection using service ORA1 will automatically login to Host2. However, there is one restriction, the SID must be the same on both machines. This feature can provide guaranteed login for application servers and for the Oracle Parallel Server.