You can use the parameter LOCAL to connect to Oracle Database without specifying a connect identifier in the connect string.

The value of the parameter LOCAL is any connect identifier, such as a net service name. For example, if the parameter LOCAL is specified as finance, you can connect to a database from SQL*Plus with:

SQL> CONNECT SMITH
Enter password: password

rather than

SQL> CONNECT SMITH@finance
Enter password: password

Oracle Net checks if LOCAL is defined as an environment variable or as a parameter in the registry, and uses finance as the service name. If it exists, then Oracle Net connects.