Each node in a Qbox cluster has an assigned public and private IP address. It also is assigned a host address, which resolves to the public IP and looks something like this:
You can use both the host address or public IP to communicate with a node from any location — within the cluster's data center or on your home/office network.
Private IP addresses have restrictions: you can only access these from within the data center in which the cluster resides (i.e. from your application servers in that region). The use of private IP addresses gives you benefits that are similar to a LAN, which dramatically reduces the latency between your application servers and a Qbox cluster.
Consider an example: if you launch an AWS cluster in the us-east-1 region, then any application servers that you have within that same region will be able to communicate directly with the cluster because it resides in the same private network.