Apache /etc/httpd/conf/httpd.conf 設定問題

[poonwaikuen]

Dear all,
I am using Red Hat Linux 8. I have set up my apache and it can show my webpage by typing http://localhost/ or http://127.0.0.1/ on my browser.
However, when I typing http://www.mydomain.com or http://(MY IP) on my browser, it show nothing.

P.S. I have registered my own IP as mydomain.com

I have changed my httpd.conf as follows:

____________________________________

ServerName mydomain.com

____________________________________

Do I need to configure other files or What wrong with my httpd.conf ?
Anybody can kindly help me! Thanks!