Introduction

Welcome back, everyone👋! I hope you’re ready to continue our exploration of Kubernetes. Today, we’re diving into one of the most fascinating - and often challenging - aspects of Kubernetes: networking.

You might be wondering, “Why do we need to understand Kubernetes networking?” While we’ve already explored how individual containers communicate, Kubernetes introduces new complexities. When you’re orchestrating hundreds of containers across multiple machines, these containers need to find and talk to each other - regardless of which machine they’re running on. Understanding how Kubernetes makes this possible is essential for anyone working with containerized applications.

By the end of this article, you’ll be able to:

• Understand how containers and pods communicate in a Kubernetes cluster
• Explain the flat network model and why it matters
• Describe how overlay networks enable cluster-wide communication

Let’s begin exploring how Kubernetes connects all these pieces together to create a working distributed system.

Kubernetes Networking Fundamentals

To begin our exploration of Kubernetes networking, let’s first understand its fundamental building block — the Pod.

What is a Pod?

A pod is the smallest deployable unit in Kubernetes. It encapsulates one or more containers that share network resources, creating a cohesive unit that runs together on the same node.

Looking at our diagram, you can see several pods, each containing one or two containers. These containers aren’t just grouped together—they’re tightly integrated through shared networking, which is fundamental to understanding how Kubernetes networking works.

When Kubernetes creates a pod:

• It first sets up a network namespace:
  A network namespace is an isolated environment provided by the operating system that gives the pod its own dedicated network stack (including its own interfaces, routing tables, and firewall rules).
• All containers in that pod share this namespace.
• The pod is automatically assigned a unique IP address.
• This IP address belongs to the pod as a whole rather than to individual containers.

This shared networking environment means all containers in a pod:

• Share the same IP address.
• Share the same network interfaces.
• Share the same port space.
• Can communicate with each other using localhost.

Understanding pods is crucial because they form the foundation of Kubernetes networking. Every networking decision in Kubernetes starts with how pods are designed to work.

Container-to-Container Communication

When containers are placed in the same pod, they form a tightly integrated unit that shares networking resources. This shared environment, enabled by the shared network namespace, allows containers to communicate easily—just as applications running on your computer communicate using localhost.

Because all containers in a pod share the same network namespace, they can exchange data without any additional configuration. For example:

• One container can run a service on port 8080.
• Another container in the same pod can access that service at localhost:8080.
• Communication is fast and reliable since it all occurs within the same network namespace.
• No special networking configuration is needed

However, this arrangement also means that containers must coordinate their port usage. Since they share the same port space, no two containers in the same pod can listen on the same port—a critical consideration when designing your application architecture.

Consider this common scenario:

• A web application container serves content on port 8080.
• A monitoring sidecar container collects metrics on port 9090.
• Both containers communicate via localhost on their respective ports, functioning as if they were individual processes on the same machine.

This pattern is particularly useful for closely related services—such as application containers paired with sidecars for logging, monitoring, or security—ensuring efficient, low-latency communication while keeping configuration simple.

Pod-to-Pod Communication

While containers within a pod communicate easily using localhost, pods themselves require a different mechanism to communicate. This is where Kubernetes’ unique approach to networking comes into play.

In our diagram, you can see pods spread across different nodes, each with its own IP address. Kubernetes automatically assigns every pod an IP address that works across the entire cluster. This means:

• Each pod is assigned an IP address that does not conflict with any other pod’s address.
• Other pods use each other’s IP addresses to communicate directly.
• The IP address stays with the pod as long as the pod is running.
• If a pod is terminated and then recreated (even under the same name), it will be assigned a new IP address.

For example, if Pod1 needs to communicate with Pod3:

• Pod1 can simply address Pod3 using Pod3’s IP address.
• This direct communication works the same way whether the pods are:
  • On the same node
  • Or on different nodes within the cluster.

This direct, IP-based communication creates a simple, consistent way for pods to exchange data regardless of their physical location in the cluster. One thing to keep in mind is the fact that a pod’s IP address is only valid for the lifetime of that pod.

In the next section, we’ll explore how Kubernetes implements this seemingly straightforward communication model through its flat networking architecture.

The Flat Network Model

Up to this point, we’ve seen how containers communicate within pods and how pods communicate with each other. Now let’s understand the networking model that makes this possible: Kubernetes’ flat network architecture.

In traditional networking, communication often follows a hierarchical path. Traffic might need to go through multiple network layers, routers, and address translations before reaching its destination. Think of a letter that needs to pass through several post offices before reaching its recipient.

Kubernetes takes a different approach. It implements a flat network model where:

• Every pod can reach every other pod directly
• No Network Address Translation (NAT) is needed
• No complex routing rules are required
• Pod IP addresses remain unchanged during communication

This flat network model brings several important benefits:

1. Simplicity

   • Pods communicate as if they’re on the same network
   • Applications don’t need to know about the underlying network topology
   • Developers can focus on application logic rather than networking details

2. Consistency

   • The same networking behavior applies across the entire cluster
   • Communication works the same way regardless of pod location
   • Applications can be moved between nodes without changing their networking code

3. Scalability

   • New nodes can be added without reconfiguring existing networking
   • Pods can be rescheduled to different nodes without breaking communication
   • The cluster can grow while maintaining the same simple networking model

Understanding Network Address Translation (NAT) in Kubernetes

As we’ve seen, Kubernetes enables direct pod-to-pod communication across the cluster. To fully appreciate this design, we need to understand why it’s different from traditional networking approaches that use Network Address Translation (NAT).

In traditional networks, NAT serves as an intermediary that modifies network traffic as it passes through. When a device sends traffic to another network:

• Its private IP address gets translated to a different address
• The translation is recorded in a NAT table
• Return traffic must go through the same NAT device
• The original address is restored using the NAT table

While NAT helps conserve IP addresses and provides some security benefits, it introduces several challenges:

• Complex routing paths
• Additional network latency
• Difficulty in tracking network flows
• Complications in direct communication between services

Kubernetes takes a fundamentally different approach. Instead of using NAT:

• Each pod gets a unique IP address that’s valid across the entire cluster
• Pods communicate directly with each other using these IP addresses
• Source and destination addresses remain unchanged during communication
• No address translation occurs as traffic moves between pods

This NAT-less design reinforces the flat network model we discussed earlier and brings several benefits:

• Simpler network architecture
• Lower network latency
• Easier troubleshooting
• More predictable communication patterns
• Simplified application design

For example, when Pod1 communicates with Pod3:

• Pod1 uses Pod3’s actual IP address
• The traffic flows directly to Pod3
• No address translation occurs along the way
• Pod3 sees Pod1’s real IP address as the source

This direct communication model is a key part of what makes Kubernetes networking both powerful and simple to understand. In the next section, we’ll explore how Kubernetes implements this design through various networking components.

Making it Work: The Implementation

We’ve explored the fundamental principles of Kubernetes networking - from pod communication to the flat network model and the elimination of NAT. Now let’s look at how Kubernetes implements these principles through a set of cooperating components.

How Kubernetes Networking Comes Together

Kubernetes achieves its networking model through the orchestration of several key components working together:

Network namespaces provide the foundation for isolation. They create separate networking environments for each pod, complete with their own interfaces, routing tables, and firewall rules. This isolation ensures that pods maintain their unique network identity.

Container runtimes like containerd and CRI-O handle the creation and configuration of these network namespaces. When a pod starts, the container runtime:

• Creates the network namespace
• Sets up virtual network interfaces
• Prepares the basic networking structure

Virtual ethernet pairs (veth) act as virtual network cables. One end connects to the pod’s network namespace, while the other connects to the node’s network, allowing pods to communicate beyond their namespace.

Container Network Interface (CNI) plugins tie everything together:

• They receive networking requests from the container runtime
• Configure pod networking according to Kubernetes requirements
• Handle IP address assignment
• Set up routing rules
• Implement the flat networking model we discussed earlier

These components work in concert to create a seamless networking experience. For example, when Kubernetes creates a new pod:

1. The container runtime creates a network namespace
2. It calls the CNI plugin to set up networking
3. The CNI plugin creates virtual ethernet interfaces
4. It assigns IP addresses and configures routing
5. The pod is now ready to communicate

This collaboration ensures that every pod gets:

• Its own network namespace
• A unique IP address
• Direct communication capability with other pods
• Connection to the node’s network

Let’s look at this process in more detail…

The Network Setup Process

Let’s walk through the detailed steps of how Kubernetes sets up networking for a new pod. Understanding this process helps clarify how all the components work together to implement the networking principles we discussed earlier.

When you create a pod, several steps occur in sequence:

1. Pod Scheduling

   • Kubernetes scheduler assigns the pod to a node
   • The node’s kubelet receives instructions to start the pod
   • Kubelet contacts the container runtime to create the pod

2. Network Namespace Creation

   • Container runtime creates a new network namespace
   • This namespace starts empty, with no interfaces or routes
   • It provides a clean networking environment for the pod

3. CNI Plugin Invocation

   • Container runtime calls the configured CNI plugin
   • Passes information about the pod and namespace
   • Requests network setup according to cluster configuration

4. Network Interface Setup

   • CNI plugin creates a virtual ethernet (veth) pair
   • One end goes into the pod’s namespace
   • Other end attaches to the node’s network
   • Configures interfaces with appropriate names and settings

5. IP Address and Routing

   • CNI plugin allocates an IP address for the pod
   • Configures the interface in the pod’s namespace
   • Sets up routes for pod-to-pod communication
   • Ensures the pod can reach other network destinations

6. Final Configuration

   • DNS settings are configured
   • Network policies are applied (if any)
   • Pod’s network stack is ready for communication

This process happens quickly and automatically, hiding the complexity from both users and application developers. The result is a pod that:

• Has its own network identity
• Can communicate with other pods
• Maintains network isolation
• Follows Kubernetes networking principles

In the next section, we’ll explore how this networking setup extends across multiple nodes through overlay networks.

Connecting the Cluster: Overlay Networks

So far, we’ve explored how networking works within a node - how pods get their IP addresses and how they communicate. But in real-world deployments, Kubernetes clusters span multiple nodes, often across different networks or even different data centers. This is where overlay networks come into play.

Why We Need Overlay Networks

The challenge of cluster networking becomes apparent when we consider the physical reality of a Kubernetes cluster:

Physical Network Constraints

• Nodes might be in different network segments
• Traditional networks don’t understand pod IP addresses
• Physical routers expect standard IP routing
• Network segments might have conflicting IP ranges

For example, imagine two nodes in different data centers:

• Node1 has Pod1 with IP 10.10.1.2
• Node2 has Pod2 with IP 10.10.1.3
• The physical network between these data centers has no knowledge of pod IP addresses
• Yet, Pod1 and Pod2 need to communicate as if they were on the same network

This creates several challenges:

• How do we maintain the flat networking model across physical boundaries?
• How do we ensure pod IP addresses remain unique across the entire cluster?
• How do we route traffic between pods on different nodes?
• How do we preserve our NAT-less direct communication?

Traditional networking solutions aren’t sufficient because:

• Physical networks don’t recognize pod IP addresses
• Network segments might not be directly connected
• Standard routing protocols don’t understand pod networking
• Physical network configuration shouldn’t need modification for Kubernetes

This is where overlay networks provide an elegant solution. They create a virtual network that spans all nodes in the cluster, hiding the complexity of the physical network underneath. In the next section, we’ll explore how overlay networks accomplish this.

How Overlay Networks Work

Overlay networks solve our cross-node communication challenges by creating a virtual network layer that sits on top of the physical network. This virtual layer ensures pods can communicate directly, maintaining our flat network model regardless of where pods are located in the cluster.

The Process of Pod-to-Pod Communication Across Nodes:

1. Encapsulation

   • When Pod1 sends traffic to Pod2 on another node
   • The overlay network captures this traffic
   • Wraps (encapsulates) the pod’s network packets inside new packets
   • These outer packets use the nodes’ real IP addresses
   • The original pod IP addresses remain unchanged inside

2. Transport

   • The encapsulated packets travel across the physical network
   • Physical routers see only the outer packet with node IPs
   • They route these packets using standard networking
   • The pod traffic is effectively “tunneled” through the physical network

3. Decapsulation

   • When packets reach the destination node
   • The overlay network removes the outer packet
   • Reveals the original pod traffic inside
   • Delivers it to the target pod
   • The pod sees direct communication with the source pod

This process is completely transparent to the pods. They continue to:

• Use their assigned IP addresses
• Communicate directly with each other
• Maintain the flat network model
• Operate without NAT

The overlay network handles all the complexity:

• Managing the virtual network
• Tracking pod locations
• Handling encapsulation/decapsulation
• Routing between nodes

Through this mechanism, overlay networks create the illusion of a single, flat network where:

• All pods can reach each other directly
• Physical network boundaries are invisible to pods
• The Kubernetes networking model is preserved
• Cluster scaling remains simple

Container Network Interface (CNI) plugins implement these overlay networks, each with its own approach to encapsulation and routing. But regardless of the specific implementation, they all achieve the same goal: enabling seamless pod-to-pod communication across the entire cluster.

Conclusion

Throughout this exploration of Kubernetes networking, we’ve uncovered how a seemingly complex system is built on clear, consistent principles. Let’s recap our journey:

We started with the fundamentals:

• Pods as the basic networking unit
• Container-to-container communication within pods
• Direct pod-to-pod communication
• The flat network model
• The elimination of NAT

We then saw how Kubernetes implements these principles:

• Network namespaces providing isolation
• Container runtimes managing network setup
• CNI plugins handling configuration
• Components working together to create pod networking

Finally, we discovered how overlay networks make it all work across a cluster:

• Creating a virtual network over physical infrastructure
• Enabling cross-node communication
• Preserving the flat network model
• Maintaining transparent pod communication

What makes Kubernetes networking elegant is not just its technical implementation, but how it shields users from complexity. Applications can communicate across a cluster as easily as they would on a single machine, while the underlying network handles the intricate details.

This foundation in Kubernetes networking prepares you for more advanced topics like network policies, service networking, and ingress controllers. But remember - all these advanced features build upon the fundamental principles we’ve explored here.

Understanding Kubernetes networking isn’t just about knowing how things work - it’s about appreciating how thoughtful design choices create a system that’s both powerful and approachable.