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the easy connect method allows you to :

Connect to a remote database without the need of a tnsnames.ora file (or other methods of resolving the location of the database).

If you know the name of the host, server, port, and service name, you can directly enter those on the command line.

the syntax is as follows:
sqlplus username@[//]host[:port][/service_name][:server][/instance_name]

For example, assuming the host name is hesta, the port is 1521, and the service name is ora12cr1, then you can connect as follows:

$ sqlplus user/pass@hesta:1521/ora12cr1

the easy connect method is handy for situations in which you’re troubleshooting connectivity issues or when you don’t have a tnsnames.ora file available (or other ways to resolve the remote connection).