How Oracle clients connect to the database

To enable a client machine to connect to an Oracle database, Oracle client software needs to be installed on the client machine, with the exception that the Java connection database can be used in JDBC Thin mode without Oracle client software. When you install an Oracle database on a machine, you do not need to have an Oracle client installed on the machine alone, because Oracle clients are installed automatically when you install the Oracle database.
Using a SQL Server database and then using Oracle's novice might have the question of asking what I don't need to install SQL Server clients when I use SQL Server? The simple reason is that SQL Server is also Microsoft, which integrates SQL Server clients into the operating system, and if Microsoft has a protocol with Oracle to integrate Oracle clients into the operating system, Then we don't have to be able to access the database on the client machine, but it seems like it's not going to happen.
Some people may ask: why is there no listening port in SQL Server, and in Oracle you want to configure the listening port? In fact, there is also a listening port in SQL Server, except that Microsoft will have the listening port fixed to 1433 and not allow you to make any changes, which gives you an illusion. There is no listening port in SQL Server, and Microsoft is encapsulating too many things in a black box, Easy to use while also bringing the need for side effects. The listening port in Oracle is directly in the configuration file, allowing random changes, but no matter how it changes, it is consistent with the listening port set by the Oracle server side.

So, what can we do to get the client machine connected to the Oracle database?
A Install related software
B Make the appropriate configuration

A Install the appropriate software in the appropriate location:
On the client machine:
1. Install Oracle's Oracle NET communication software on the client machine, which is included in Oracle's client software.
2. Sqlnet.ora files are correctly configured:
NAMES. Directory_path = (TNSNAMES, ...)
NAMES. Default_domain=db_domain
In general, we do not use names.default_domain parameters. If you want to use the # annotation for this parameter or delete it, you can use the default value for the Names.directory_path parameter, and sometimes you need to comment out the Names.default_domain parameter, which is explained in detail below.
3. Tnsname.ora files are properly configured

On the server-side machine:
1. Ensure listener has been activated.
2. Ensure that the database is started.
If the database is not started, use:
Oracle 9i:
Dos>sqlplus "/As SYSDBA"
Sqlplus> Startup
Oracle 8i:
Dos>svrmgrl
Svrmgrl>connect Internal
Svrmgrl>startup
command to start the database
If listener does not start, use:
Lsnrctl start [listener name]
LSNRCTL status [listener name]
Command Start Listener