When the server is directly connected to the internet, the firewall is to be setup for security purpose. On CentOS 7, FirewallD is a kind of minimum requirements in terms of security point of view. You can simply install FirewallD by sudo yum install firealld and add policy by sudo firewall-cmd --zone=public --permanent --add-service=http

STEP 1. Install FirewallD

sudo yum install -y firewalld

STEP 2. Enable FirewallD

sudo systemctl enable firewalld
sudo reboot

STEP 3. Verify that the service is running and reachable

sudo firewall-cmd --state