When you create a database link with a name for example "TEST" then you might notice that oracle adds an extra ".WORLD" or something else.
This extra addition comes from the GLOBAL_NAME of the database.
To see what is the GLOBAL_NAME of your database, execute:
SELECT * FROM GLOBAL_NAME;
To change the global_name of you database execute:
ALTER DATABASE RENAME GLOBAL_NAME TO NEW_NAME;
Then if you create a new database link with name XXX, then oracle will add the extra "NEW_NAME", so it will be XXX.NEW_NAME
Be careful, there is the also a parameter called "global_names":
SELECT * FROM V$PARAMETER WHERE NAME = 'global_names';
Usually takes a value of FALSE.
Tip: When GLOBAL_NAMES=TRUE then the db link name MUST be identical to the global_name of the db to which it connects.