K8s defines an interface with requirements (such as every pod must be directly addressable by ip from every other pod) and you bring your own networking implementation. It's never just the standard container networking though as it needs to span horizontally.
Kubernetes does not use the traditional Docker networking model, but rather implements its own networking model. The Kubernetes networking model ensures that every pod gets its own IP address and allows pods to communicate with each other without NAT (Network Address Translation).
So there used to be something called "kubenet" which was built-in implementation that worked similar to what this article described, nowadays all networking is out-of-tree and handled by external networking plugins via the CNI[0].
