Flannel
The original "simple Kubernetes CNI" — gives each node a subnet and ships pod traffic between nodes over a VXLAN overlay.
Created by CoreOS in 2014. Default backend is vxlan (pod packets encapsulated in UDP); alternatives are host-gw (needs L2 connectivity but avoids overlay overhead) and wireguard. No NetworkPolicy support out of the box.
Competitors: Cilium (eBPF-based, policy + observability), Calico (BGP routing, policy). Flannel survives where simplicity wins — k3s default, embedded clusters, home servers.