How to connect SQLPlus without tnsnames.ora

Connect using one of service_name:

sqlplus username/password@//host[:port][/service_name]

where service_name can be one of simple name or domain name specified in service_names parameter:

show parameter service_names
ALTER SYSTEM SET service_names = 'ORCL', 'us.acme.com' COMMENT='Add new services' SCOPE=BOTH;