1.- Check the domain in your sqlnet.ora file located in the server
2.- Enter the entry to point to another database in tnsnames.ora file
3.- Check the resolution of the service with and without the domain alias with tnsping utility
4.- Create the database link ( It recommendable that the name of the database link be the same name of the queried database to avoid more details with global_names value )
CREATE DATABASE LINK
Use the CREATE DATABASE LINK statement to create a database link. A database link is a schema object in one database that enables you to access objects on another database. The other database need not be an Oracle system. However, to access non-Oracle systems you must use Oracle Heterogeneous Services.
Once you have created a database link, you can use it to refer to tables and views on the other database. In SQL statements, you can refer to a table or view on the other database in by appending @dblink to the table or view name. You can query a table or view on the other database with the SELECT statement. You can also access remote tables and views using any INSERT, UPDATE, DELETE, or LOCK TABLE statement.