Oracle Message NNL-00024 - warning: no preferred name servers in SQLNET.ORA

ERRORwarning: no preferred name servers in SQLNET.ORA
CAUSEThe control program was unable to find any preferred name servers in SQLNET.ORA, and will therefore not connect to a default server.
ACTIONMake sure that the SQLNET.ORA file contains a valid names.preferred_servers entry.