To build reliable, distributed systems, components must be able to communicate with each other consistently. In the Kubernetes environment, where Pods can be created, destroyed, and rescheduled onto different nodes at any moment, relying on their transient IP addresses for communication is not a sustainable strategy. To solve this, Kubernetes establishes a set of fundamental networking rules that govern how resources interact within the cluster. This networking model provides a consistent and predictable environment for your applications, abstracting away the complexity of the underlying infrastructure.
The most important principle of the Kubernetes network model is that every Pod is assigned its own unique IP address. From the perspective of the containers running inside a Pod, they are in their own isolated network environment. They can bind to any port they choose on their localhost address, and they see the Pod's IP address as their own.
This model is significantly different from other container networking approaches, like the default Docker bridge network, which often require you to map ports from the container to the host machine. By giving each Pod a real IP address, Kubernetes removes this layer of complexity. A Pod behaves much like a virtual machine or a physical host on a network; it has an IP address that other Pods can use to communicate with it directly.
This design choice simplifies application development and deployment. You no longer need to coordinate port usage across different applications running on the same host, and applications can be migrated from VMs to containers with minimal changes to their network configuration.
Building on the IP-per-Pod foundation, Kubernetes mandates a flat network space where three conditions are met:
This creates a clean, straightforward networking environment. If your application is running in a Pod with the IP 10.244.1.5, it can open a connection directly to another application in a Pod at 10.244.2.10, regardless of where those Pods are physically running in the cluster.
Pods on different nodes communicate directly using their unique IP addresses. The underlying network fabric handles the routing required to connect them.
You might be wondering how this flat network is actually created. Kubernetes itself does not implement the networking layer. Instead, it relies on a standard plugin specification called the Container Network Interface (CNI).
When you set up a Kubernetes cluster, you must choose and install a CNI plugin. This plugin is responsible for the heavy lifting:
Popular CNI plugins include Calico, Flannel, Weave Net, and Cilium. Each uses different techniques (such as overlay networks or BGP) to implement the Kubernetes networking model, but they all adhere to the same set of rules. This allows you to choose a network provider that best fits your performance, security, and operational requirements without changing how your applications are written.
This model provides a powerful foundation for communication. Every Pod has a unique, routable IP address. However, this raises a new and important question.
In previous chapters, we established that Pods are ephemeral. A Deployment controller can terminate a Pod and create a new one to replace it at any time. When a new Pod is created, it will be assigned a new IP address.
If you have a frontend application that needs to talk to a backend API, you cannot hardcode the backend Pod's IP address. That IP address is not stable. How does the frontend discover the current, valid IP address for a healthy backend Pod?
This problem of service discovery is precisely what the Kubernetes Service object is designed to solve. While the networking model gives us connectivity, we need a higher-level abstraction to provide a stable endpoint for a group of ever-changing Pods. In the next section, you will learn how Services accomplish this.
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