Unable to mount oracle instance I am trying to shutdown the system, then I want to mount it. I have tried this:

SQL&gt; shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL&gt; startup mount;
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
SQL&gt; startup mount;
ORA-24324: service handle not initialized
ORA-01041: internal error. hostdef extension doesn't exist


I have googled for solution but I can't found the exact reason that causes it to go wrong.
Exit sqlplus.

Make sure your os user is part of the DBA group (ORA_DBA in Windows).

Make sure ORACLE_SID is set to the SID of the database and unset TWO_TASK:

Windows:

set ORACLE_SID=ORCL
set TWO_TASK=


or

Unix/Linux

ORACLE_SID=ORCL
TWO_TASK=
export TWO_TASK


Now, connect as sysdba:

sqlplus / as sysdba


Now, startup the database:

SQL&gt; startup mount;

SQL&gt; alter database open;