Each of Sucuri comes with a firewall service. To enable firewall protection to your website, you have to point the site domain to Sucuri firewall IP in the domain DNS zone.
Follow these steps to get the Firewall IP.
Log in to your client area portal.
Click My Services >> My Products/Services
Click Sucuri service
4. Get your Firewall IP here.
If your domain name is with other providers, you may get the assistance from them to update the domain name A record to the Firewall IP.
The Sucuri product page in your client area includes more of these detailed DNS instructions for your reference, including a DNS propagation checking tool.