Add docs page for IPv6-only (#756)

---------

Co-authored-by: Louise K. Schmidtgen <louise.schmidtgen@canonical.com>

docs/src/snap/howto/networking/index.md

@@ -16,4 +16,5 @@ how to configure and use key capabilities of {{product}}.
 /snap/howto/networking/default-ingress.md
 /snap/howto/networking/default-loadbalancer.md
 /snap/howto/networking/dualstack.md
+/snap/howto/networking/ipv6.md
 ```

docs/src/snap/howto/networking/ipv6.md

@@ -0,0 +1,142 @@
+# How to Setup an IPv6-Only Cluster
+
+An IPv6-only Kubernetes cluster operates exclusively using IPv6 addresses,
+without support for IPv4. This configuration is ideal for environments that
+are transitioning away from IPv4 or want to take full advantage of IPv6's
+expanded address space. This document, explains how to set up
+an IPv6-only cluster, including key configurations and necessary checks
+to ensure proper setup.
+
+## Prerequisites
+
+Before setting up an IPv6-only cluster, ensure that:
+
+- Your environment supports IPv6.
+- Network infrastructure, such as routers, firewalls, and DNS, are configured
+to handle IPv6 traffic.
+- Any underlying infrastructure (e.g. cloud providers, bare metal setups)
+must be IPv6-compatible.
+
+## Setting Up an IPv6-Only Cluster
+
+The process of creating an IPv6-only cluster involves specifying only IPv6
+CIDRs for pods and services during the bootstrap process. Unlike dual-stack,
+only IPv6 CIDRs are used.
+
+1. **Bootstrap Kubernetes with IPv6 CIDRs**
+
+Start by bootstrapping the Kubernetes cluster and providing only IPv6
+CIDRs for pods and services:
+
+```bash
+sudo k8s bootstrap --timeout 10m --interactive
+```
+
+When prompted, set the pod and service CIDRs to IPv6 ranges. For example:
+
+```
+Please set the Pod CIDR: [fd01::/108]
+Please set the Service CIDR: [fd98::/108]
+```
+
+Alternatively, these values can be configured in a bootstrap configuration file
+named `bootstrap-config.yaml` in this example:
+
+```yaml
+pod-cidr: fd01::/108
+service-cidr: fd98::/108
+```
+
+Specify the configuration file during the bootstrapping process:
+
+```bash
+sudo k8s bootstrap --file bootstrap-config.yaml
+```
+
+2. **Verify Pod and Service Creation**
+
+Once the cluster is up, verify that all pods are running:
+
+```sh
+sudo k8s kubectl get pods -A
+```
+
+Deploy a pod with an nginx web-server and expose it via a service to verify
+connectivity of the IPv6-only cluster. Create a manifest file
+`nginx-ipv6.yaml` with the following content:
+
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-ipv6
+spec:
+  selector:
+    matchLabels:
+      run: nginx-ipv6
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        run: nginx-ipv6
+    spec:
+      containers:
+      - name: nginx-ipv6
+        image: rocks.canonical.com/cdk/diverdane/nginxipv6:1.0.0
+        ports:
+        - containerPort: 80
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: nginx-ipv6
+  labels:
+    run: nginx-ipv6
+spec:
+  type: NodePort
+  ipFamilies:
+  - IPv6
+  ports:
+  - port: 80
+    protocol: TCP
+  selector:
+    run: nginx-ipv6
+```
+
+Deploy the web-server and its service by running:
+
+```sh
+sudo k8s kubectl apply -f nginx-ipv6.yaml
+```
+
+3. **Verify IPv6 Connectivity**
+
+Retrieve the service details to confirm an IPv6 address is assigned:
+
+```sh
+sudo k8s kubectl get service nginx-ipv6 -n default
+```
+
+Obtain the service’s IPv6 address from the output:
+
+```
+NAME         TYPE       CLUSTER-IP    EXTERNAL-IP   PORT(S)        AGE
+nginx-ipv6   NodePort   fd98::7534    <none>        80:32248/TCP   2m
+```
+
+Use the assigned IPv6 address to test connectivity:
+
+```bash
+curl http://[fd98::7534]/
+```
+
+A welcome message from the nginx web-server is displayed when IPv6
+connectivity is set up correctly.
+
+## IPv6-Only Cluster Considerations
+
+**Service and Pod CIDR Sizing**
+
+Use `/108` as the maximum size for Service CIDRs. Larger ranges (e.g., `/64`)
+may lead to allocation errors or Kubernetes failing to initialize the IPv6
+address allocator.