I've mounted my database,when i give the follwing query,'Select * from v$database;",it works,but when i give the query,"select * from dba_data_files;",it gives the following error,

ORA-01219:database not open:queries allowed on fixed tables/views only.

Just clear me,
1. What are the tables/views internally marked as fixed by ORACLE & on what basis ORACLE does this marking?
2. What is meant by fixed table/view?