In the kubeadm-config.yaml there is a section for networking. Change the pod subnet and service subnet. Below is the default config for flannel since we are going to use flannel as Container Network Interface(CNI). Make sure these subnets do not coincide with the subnets that exist in your external network. These are the default configuration. You can change as you see fit for your environment.

networking:

dnsDomain: cluster.local

podSubnet: 192.168.100.0/20

serviceSubnet: 10.225.101.0/24