ORACLE - How connect to DB

Typical form is: username/password@db_instance

connect scott/tiger@orcl

When is db_instance set in registry, you can use directly only username and password:

connect scott/tiger

When connection is OK:

SQLWKS> connect scoot/tiger@orcl
ORA-01017: invalid username/password; logon denied
SQLWKS> connect scott/tiger@orcl
Connected.