Select DBID from v$database;
In situations requiring the recovery of your server parameter file or control file from autobackup, you need to know the DBID. You should record the DBID along with other basic information about your database.
If you do not have a record of the DBID of your database, then you can find it in the following places without opening your database:
1. The DBID is used in forming the filename for the control file autobackup. Locate this file, and then you can determine the DBID from the control file.
My control file backup is : c-16509508-20100804-00.
So in my case the DBID is 16509508
2. If you have any text files that preserve the output from an RMAN session, then the DBID is displayed by the RMAN client when it starts up and connects to your database. Typical output follows:
% rman TARGET /
Recovery Manager: Release 126.96.36.199.0 - Production on Wed Aug 01 17:51:30 2010
Copyright (c) 1982, 2007, Oracle. All rights reserved.
connected to target database: PROD (DBID=16509508)