Network in Database

Listener --> server side configuration file --> listener.ora
TNSnames --> Client side configuration file --> tnsnames.ora

listener.ora

it contains port.
on which port the database is listening.
protocol --> TCP ( on which protocol its running )
hostname or IP address
All these can assigned to Listener name, so that we can start and stop the listener ( Alias Name )
location --> TNS admin ? --> network configuration location.
This network location will be in $ORACLE_HOME/network/admin
Listener Base --> Listener related logs will go here, its like a base directory.
lsnrctl --> listener Control

lsnrctl start name_of_the_listener
lsnrctl status name_of_the_listener

Change the listener port , now the scenario will be like --> Listener is running in 1522 but the LREG takes the details of DB and then search for 1521 port. Even if you wait for 60 secs it wont happen . So your Listener will never get the update of DB details. " The Listener supports no service " --> That means Listener doesn't have any DB details.
show parameter local_listener;
alter system set local_listener = ' ( Address = ... ) ' ;
Now my LREG is aware where my listener is running.

Static Registration

Static registration with SID OR Service name.
Forcing the listener to register the DB details ( don't worry about the status of the DB , but just register )
UNKNOWN is the static registration.
What is the purpose of registering with Static registration ? --> Its used in Data Guard Build for online mode is one of the use case.

tnsnames.ora

Its a client side configuration file.
The client may connect from any application.
Inside the file we need : Port , protocol , hostname or IP address , service name ( name of the DB will be equal to service name , most of the time )
All these 4 details are required.
The client will connect with these parameters along with username and password. -Each time all these details to be passed , to avoid this , Oracle came up with ALIAS NAME in the tnsnames.ora file which is inside the network admin file.

sqlnet.ora

Both server and client side file.
It can be used for many purposes.
If there is any miss-match in version at any side , client or server side , at that time this HERO will come into picture. OR at TDE / Wallet usage.
In simple terms , its used in : ( search in internet "sqlnet.ora" )

lower client version wallet location AD/LDAP Connect Timeout ( Client to server connectivity ) 40 or > 40 parameters can be defined.

NOTE

Oracle_home will be more or less like --> /u01/app/oracle/product/19.0.0.1/dbhome_1
Listener supports no service.
For every 60 secs these pmon goes to listener and registers its details. what details ? DB details . This is as per 11g.
LREG in 12c --> Listener register , every 60 secs it registers the DB details with the LISTENER. This registration happens only when the DB is running with 1521 PORT.
Listener is a dynamic parameter.
What is UNKNOWN ? when we check the listener status ? [TBD] --> Ready or Unknown. READY --> Dynamic Registration , UNKNOWN --> Static Registration --> we have the DB details in the listener configuration file and forcing the listener to register the DB details and don't worry about the DB status.

Commands

su - oracle
. oraenv
env | grep ORA
ps -ef|grep tns
ps -ef|grep lreg
sqlplus / as sysdba
telnet host 1521
netstats -anp | grep 1521
ps -ef|grep smon
startup
sqlplus user/password
sqlplus user/password@"(DESCRIPTION ...)
select instance_name from v$instance;
show user;
sqlplus user/password@alias_name;