Before a JDBC application connects to a database, it must cause the proper JDBC driver to be loaded in the Java session.
For embedded environments, when Derby runs in the same JVM as the application. This is commonly referred to as the embedded drvier.
For client/server environments that use the Derby Network Server. This is commonly referred to as the Network Client driver.
You can use ij to connect to any database that supplies a JDBC driver. For those databases, you would need to load the supplied JDBC driver.