You can leave many connection parameters unspecified, and instead use the default behavior to make a connection.
If you are connecting to a database on your local server, and more than one database has been started on that server, you need to specify the database you wish to connect to, but you can leave the server as a default:
dbn=db_name uid=user_id pwd=password
If more than one server is running, you need to specify which one you wish to connect to. If only one database has been started on that server, you do not need to specify the database name. The following connection string connects to a named server, using the default database:
eng=server_name uid=user_id pwd=password