I'm not sure if you're focusing on bare metal, but Kubernetes can assign IPs itself, and does so out-of-the-box on AWS, GCE, GKE, Azure & OpenStack (and probably others as well). On AWS it uses the equivalent to flannel's aws-vpc by default (and one of my personal goals is to make it easy to install one of the networking options that goes to bigger scale, so I greatly appreciate your guide here!)
So you don't need a third party networking solution, but Kubernetes is open so a number have been nicely integrated. flannel is the most popular choice, but there are many others (I hear about Calico a lot)
So you don't need a third party networking solution, but Kubernetes is open so a number have been nicely integrated. flannel is the most popular choice, but there are many others (I hear about Calico a lot)