An overlay network is not a requirement. The only requirement is that pods (collection of containers) should be able to communicate with each other directly wit

An overlay network is not a requirement. The only requirement is that pods (collection of containers) should be able to communicate with each other directly without NAT. Each pod gets an IP address in the container network. Overlay network technologies (flannel, weave, calico, etc) are popular but they aren't mandatory. You can implement it using hardware switches and VLANs if you wish.