Kubernetes Networking: Understanding the Network Model

Question: “How do Pods, nodes, and Services communicate within a Kubernetes cluster?”

Kubernetes networking is built on the network model, which defines how Pod-to-Pod, Pod-to-Service, and external-to-internal communication works.
This model involves components like CNI plugins (Flannel, Calico), CoreDNS, and service discovery mechanisms.

In this post, you’ll learn:

• The core concepts of the Kubernetes network model
• How Pods communicate with each other
• Node-to-node networking and the role of CNI plugins
• How Services and DNS interact with Pods

Table of Contents

1. What Is the Kubernetes Network Model?
2. Pod-to-Pod Communication
3. Node-to-Node Networking
4. What Is CNI (Container Network Interface)?
5. Services and DNS Integration
6. Hands-On: Testing Pod Communication
7. FAQ (Answer Engine Optimization)
8. Key Takeaways
9. Final Thoughts

1. What Is the Kubernetes Network Model?

Kubernetes follows a simple but powerful rule:

Every Pod must have its own IP address, and all Pods must be able to communicate with each other without NAT (Network Address Translation).

This principle allows Pod-to-Pod, Pod-to-Service, and external communication to work seamlessly across the cluster.

2. Pod-to-Pod Communication

• Each Pod gets a unique IP address.
• Within the same node: Communication happens via a bridge network.
• Across nodes: Traffic is routed via CNI plugin-managed routes.

3. Node-to-Node Networking

• kubelet and kube-proxy manage traffic routing between Pods.
• Overlay networks (Flannel, Weave Net, Calico) or routing-based solutions ensure connectivity across nodes.

4. What Is CNI (Container Network Interface)?

CNI defines how network interfaces should be created for containers.
Popular CNI plugins include:

• Flannel: Simple overlay network.
• Calico: Advanced networking with policy support.
• Weave Net: Automated peer-to-peer networking.

Example installation (Flannel):

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kubectl apply -f https://raw.githubusercontent.com/flannel-io/flannel/master/Documentation/kube-flannel.yml

5. Services and DNS Integration

• Services provide a stable endpoint despite Pod IP changes.
• CoreDNS automatically assigns DNS names to Services.

Example:

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curl http://my-service.default.svc.cluster.local

6. Hands-On: Testing Pod Communication

Step 1: Create two Pods

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kubectl run pod-a --image=nginx
kubectl run pod-b --image=busybox --command -- sleep 3600

Step 2: Test communication from pod-b to pod-a

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kubectl exec -it pod-b -- wget -qO- http://<pod-a-ip>

7. FAQ (Answer Engine Optimization)

Q1. Why can’t Pods be accessed directly from the internet? A. Pods are part of an internal cluster network and are not directly exposed.

Q2. Is a CNI plugin required? A. Yes, CNI plugins are necessary for Pod networking in most Kubernetes setups.

Q3. What should I check if Pod-to-Pod communication fails? A. Verify CNI plugin status, Network Policies, and iptables rules.

8. Key Takeaways

9. Final Thoughts

Understanding the Kubernetes network model helps you design reliable and secure service communication.