Each client has its own tnsname.ora on the local PC. Let say I want to connect to a new location instead from the tnsname. How can I connect?

I used 'sqlplus system/admin@172.20.21.15:1521rcl' where orcl is the service name. I got an error ORA-06401: NETCMN: invalid driver designator. Pls help on how to connect without a tnsname entry. Tks.