After the master of the cluster is ready to handle jobs and the services are running, for the purpose of making containers accessible to each other through networking, we need to set up the network for container communication. It is even more important initially while building up a Kubernetes cluster with kubeadm, since the master daemons are all running as containers. kubeadm supports the CNI (https://github.com/containernetworking/cni). We are going to attach the CNI via a Kubernetes network add-on.

There are many third-party CNI solutions that supply secured and reliable container network environments. Calico (https://www.projectcalico.org), one CNI provide stable container networking. Calico is light and simple, but still well implemented by the CNI standard and integrated with Kubernetes:

$ kubectl apply -f https://docs.projectcalico.org/v2.6/getting-started/kubernetes/installation/hosted/kubeadm/1.6/calico.yaml

Here, whatever your host OS is, the command kubectl can fire any sub command for utilizing resources and managing systems. We use kubectl to apply the configuration of Calico to our new-born Kubernetes.

More advanced management of networking and Kubernetes add-ons will be discussed in Chapter 7, Building Kubernetes on GCP.