Change the Port number after the Installation

How to change the Port number after the installation of Oracle software?

Ports 1521/1526 are the default port numbers Oracle will listen on for SQL*Net (Oracle 7) or Net8 (Oracle 8) out of the box.

There is a file in directory:

%ORACLE_HOME%\network\admin (Oracle 7) or %ORACLE_HOME%\net8\admin(Oracle 8) called listener.ora that specifies the ports to listen on.

(if you change it you need to change the tnsnames.ora file on all clients that connect to it - including itself, that file usually resides in the same location- if you don't they will not know which port to try to connect to)