Is it possible to use private ip address instead of domain for nginx load balancer setup?

Hi, I try to create one nginx load balancer and 1 apache php backend server by refer to nginx load balancing.

I want to limit all public access to the backend server. Only load balancer can accept public access (like http or https). Then I want my load balancer pass the query to the backend server via private network.

So I setup both servers with private network. I successfully query the nginx welcome page from load balancer. I also successfully get the apache welcome page from backend before I restrict public access with digitalocean firewall.

Then I setup the upstream directive for nginx like what it showed from the article except I use the backend server’s Private IP instead of a domain name in config file like:

upstream backend{ server 10.130.20.01; <== in the article this is backend1.example.com } server { … location / { proxy_pass http://backend; } }

At this time I assume I should see the apache page when I visit the load balancer public IP but it did not show up. I wonder if this is because of the backend private IP which I place in the nginx config file? Is it possible to use ip instead of domain to setup nginx load balancer? Thanks