$sudo a2enmod ssl
$sudo a2ensite default-ssl

modify /etc/apache2/sites-available/default-ssl.conf

$sudo systemctl restart apache2

check open ports:

$nc -zv young.hanyang.ac.kr 80
$netstat -tlnp
$nmap -sT hostname
$sudo lsof -nP -iTCP -sTCP:LISTEN

$sudo ufw status
$sudo ufw help