GLOBAL_NAMES specifies whether a database link is required to have the same name as the database to which it connects.
GLOBAL_NAMES is either set to FALSE OR TRUE.

If the value of GLOBAL_NAMES is false, then any name can be given to DB_LINK. If value is TRUE, then database link name should be same as that of the database it is pointing.

Default value is FALSE:

Lets create a database link with any name:

So with GLOBAL_NAME set to FALSE, db_link name is independent of target database name.

Update the GLOBAL_NAMES value to TRUE:

Create database link as before(with different name)

We can see, when global_name is TRUE,DB_LINKs with different name as that of database name is not working.

Lets create a db_link having same name as target db.

Voilla!!! Working fine.