Cloudflare blocking my backend server IP range?

HI,
I have an ip range, where my websites use cloudflare, I will for example a specific IP:
123.31.43.239
At that IP, we can directly access http://123.31.43.239 at port 80
But if using an arbitrary domain that is proxy via cloudflare intermediary to 123.31.43.239, we cannot access
Example: http://duytest.voduy.com/
(You can point any subdomain to my IP to check)

What is the problem? Problems from my ISP or Cloudflare network that has limited range of networks?

What error message are you getting?

You can also Allow those addresses in Firewall -> Tools