SQL> create table t1 as select * from dba_tables;

Table created.

SQL> exec show_space('T1');
****************************************************
Free Blocks.............................0
Total Blocks............................48
Total Bytes.............................196608
Unused Blocks...........................5
Unused Bytes............................20480
Last Used Ext FileId....................10
Last Used Ext BlockId...................801
Last Used Block.........................11
****************************************************

PL/SQL procedure successfully completed.

SQL> alter table t1 allocate extent;

Table altered.

SQL> exec show_space('T1');
****************************************************
Free Blocks.............................0
Total Blocks............................64
Total Bytes.............................262144
Unused Blocks...........................21
Unused Bytes............................86016
Last Used Ext FileId....................10
Last Used Ext BlockId...................801
Last Used Block.........................11
****************************************************

PL/SQL procedure successfully completed.

SQL> alter table t1 deallocate unused;

Table altered.

SQL> exec show_space('T1');
****************************************************
Free Blocks.............................0
Total Blocks............................48
Total Bytes.............................196608
Unused Blocks...........................5
Unused Bytes............................20480
Last Used Ext FileId....................10
Last Used Ext BlockId...................801
Last Used Block.........................11
****************************************************

PL/SQL procedure successfully completed.

SQL> alter table t1 allocate extent(instance 1);

Table altered.

SQL> exec show_space('T1');
****************************************************
Free Blocks.............................21
Total Blocks............................64
Total Bytes.............................262144
Unused Blocks...........................0
Unused Bytes............................0
Last Used Ext FileId....................10
Last Used Ext BlockId...................817
Last Used Block.........................16
****************************************************

PL/SQL procedure successfully completed.

SQL> alter table t1 deallocate unused;

Table altered.

SQL> exec show_space('T1');
****************************************************
Free Blocks.............................21
Total Blocks............................64
Total Bytes.............................262144
Unused Blocks...........................0
Unused Bytes............................0
Last Used Ext FileId....................10
Last Used Ext BlockId...................817
Last Used Block.........................16
****************************************************

PL/SQL procedure successfully completed.