Kubernetes doesn't have a "default" as such. It requires something external to manage the subnet, and needs to be configured to use it.

However, if you run it on AWS, it can automatically configure a bridge (cbr0) and configure up the VPC routing table for you.

GCE (Google's managed Kubernetes on Google Cloud) also handles this automatically.

There's also experimental support for Flannel built into K8s, which can be enabled with a flag. Not sure if it's worth using.