TNS文件里有多个ADDRESS地址是什么意思求助

MESDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.81)(PORT = 1521))
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.82)(PORT = 1521))
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.83)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = RAC)
    )
  )里面有三个ADDRESS 是什么意思呢  有什么作用