12.2 DBLINK Feature Changes - Global or implict DBLINKS
Prior to version 12.2, it was possible to create an implicit or global dblink if the origin or source database referenced an LDAP naming directory.
For example, you could select data across a non-existent dblink as long as the link name was an LDAP resolvable service:
select * from dual@mydb
If 'mydb' could be resolved as a valid connect string via LDAP naming from the source database, this would
behave like any other dblink and retrieve data from the target specified in the string : mydb
See this knowledge base document for more information on this feature.
What are Global Database Links and How do you Disable them? (