About Listener.ora

Listener.ora

Listener is a utility and act as communication bridge between clients and oracle server, the configuration file resides on the server ,where oracle database installed .

Can configure multiple address ( like multiple ADDRESS entries with different hosts.,port etc..)

PMON register DB_NAME,SID,SERVICE_NAME with the listener dynamically this case listener.ora file like below ( this process call service registration)

NOTE: By default the SERVICE_NAME parameter is set to SID, but we can set multiple values for parameter like below 

SQL>alter system set SERVICE_NAME=DB11G,DB12G

now you can create new Service Naming using DB12G as well (using netmgr), this will store tnsname.ora file

LISTENER =

 (DESCRIPTION_LIST =

    (DESCRIPTION =

     (ADDRESS = (PROTOCOL = TCP)(HOST = oel5box)(PORT = 1521))

    )

 )

Or we can stored these information statistically in the listener.ora and that file look likes below

LISTENER =

 (DESCRIPTION_LIST =

    (DESCRIPTION =

     (ADDRESS = (PROTOCOL = TCP)(HOST = oel5box)(PORT = 1521))

    )

 )

SID_LIST_LISTENER =

 (SID_LIST =

    (SID_DESC =

     (GLOBAL_DBNAME = DB11G)

     (SID_NAME = DB11G)

     (ORACLE_HOME = /u01/app/oracle/product/11.1.0/db_1)

    )

 )