You have heard about flannel in the context of container orchestration. What is flannel?
"Flannel is a simple and easy way to configure a layer 3 network fabric designed for Kubernetes." Taken from https://github.com/coreos/flannel
"Flannel is a networking technology used to connect Linux Containers. It is distributed and maintained by CoreOS, the producer of the stripped-down CoreOS Linux operating system for containers, as well as the rkt container system that competes with Docker." Taken from
"Flannel is a very simple overlay network that satisfies the Kubernetes requirements." taken from https://kubernetes.io/docs/concepts/cluster-administration/networking/