Listen command identifies the ports on which the Web server accepts incoming requests. By default, the Apache HTTP Server is set to listen to port 80 for non-secure Web communications and (in the
/etc/httpd/conf.d/ssl.conf file which defines any secure servers) to port 443 for secure Web communications.
If the Apache HTTP Server is configured to listen to a port under 1024, only the root user can start it. For port 1024 and above,
httpd can be started as a regular user.
Listen directive can also be used to specify particular IP addresses over which the server accepts connections.