The database ID or DBID is a unique identification number computed when the database is created.
To find out this number, you can either:
1) Login to RMAN, it will display the DBID upon connection to the target database.
e.g. shell> rman target /
2) You can obtain the DBID by querying V$DATABASE
eg. SQL> select dbid from v$database;
If the database is down and you can’t access either of these two methods. There is a third way.
Assuming you have some RMAN backups for the database, the DBID is the first big number of the backup file.
For instance, if the file name was c-1235460798-20100929-01 then the DBID is 1235460798