How to whitelist visitor traffic from cloundflare to my Web server


I have sub domain created in Cloudflare and pointed to my Web server IP address.
I don’t want to open public to access my Web server as default.
I want to limit Web server to whitelist IP from Cloudflare.
How to do it?
Thank you

Are you asking how to configure your system firewall to only accept connections from Cloudflare? That is system administration related and in that case rather off-topic for here. StackExchange should have more on that.