When you do not specify a service name (alias) Oracle assumes you mean the local instance specified by the ORACLE_SID value in the registry. Sambavan is correct, I would guess that Oracle816 is not the NT node name of the computer you are trying to connect to. Compare the local TNSNAMES.ORA file with one on a machine which does work. If that is the only alias in the TNSNAMES.ORA file, just copy one from a working machine.