Despite the value of the db_domain is null.I still cannot change global_name.
What should I do?

SQL> SELECT * FROM GLOBAL_NAME;

GLOBAL_NAME
--------------------------------------------------------------------------------
ORATEMP.REGRESS.RDBMS.DEV.US.ORACLE.COM

SQL> ALTER DATABASE RENAME GLOBAL_NAME TO oratemp ;

SQL> SELECT * FROM GLOBAL_NAME;

GLOBAL_NAME
--------------------------------------------------------------------------------
ORATEMP.REGRESS.RDBMS.DEV.US.ORACLE.COM

You just renamed it to what was already being used.
http://download.oracle.com/docs/cd/B19306_...in.htm#i1007592

so what should I do to change the global_name to instance name.
Because of this long global name link name appears like:


CREATE PUBLIC DATABASE LINK ASNET
CONNECT TO ASNET
IDENTIFIED BY <pwd>
USING 'RAC1';


ASNET.REGRESS.RDBMS.DEV.US.ORACLE.COM

This clown is a forum spammer:

http://www.dbforums.com/showthread.php?p=6347537

I have one last question.
During the installation of a database, what things should I consider, inorder not to see global_name like below.


SQL> select * from global_name;

GLOBAL_NAME
--------------------------------------------------------------------------------
ATM.REGRESS.RDBMS.DEV.US.ORACLE.COM