In order to provide a common medium for containers to communicate with each other, OpenShift makes use of an overlay network that's implemented via VXLAN. The Virtual eXtensible Local Area Network (VXLAN) protocol provides a mechanism for transferring Layer 2 (Ethernet) frames across Layer 3 (IP) networks. Depending on the SDN plugin being used, the scope of communication may be limited to pods within the same project or maybe completely unrestricted. No matter which plugin is used, the network topology is still the same.
When a new node is registered in etcd, the master allocates a private /23 subnet from the cluster network. By default, subnets are allocated from 10.128.0.0/14, but can be configured in the networkConfig stanza of the master configuration file. The following is an excerpt from the file containing the relevant parameters...