RAC - force connection to one instance using tnsnames

We had an issue where the firewall was only open to one node of the RAC. server load balancing in the listeners meant we couldn;t connect at all even though the firewall was open to the one node. The short term fix while we got the firewall opened was to specify the instance specifically in the tnsnames entry

DB.WORLD =

  (DESCRIPTION =

    (ADDRESS_LIST =

      (ADDRESS = (PROTOCOL = TCP)(HOST = xxxxx)(PORT = 1521)))

    (CONNECT_DATA =

      (SERVICE_NAME = DB)

      (INSTANCE_NAME = DB1))

  )