Kubernetes Pod CIDR Sizing
Size the pod CIDR for Kubernetes clusters. Covers pods per node, max nodes, CNI plugins, and how to calculate the minimum CIDR block for your cluster.
Detailed Explanation
Sizing the Pod CIDR for Kubernetes
Every Kubernetes pod needs a unique IP address. The pod CIDR must be large enough to accommodate all pods across all nodes in the cluster. Getting this wrong leads to pod scheduling failures.
The Math
Total pod IPs needed = max_nodes x max_pods_per_node
Example: 100 nodes x 110 pods/node = 11,000 IPs
Minimum CIDR: /18 (16,384 IPs)
Default Configurations
| Platform | Default Pod CIDR | Max Pods/Node |
|---|---|---|
| kubeadm | 10.244.0.0/16 | 110 |
| EKS (VPC CNI) | VPC CIDR | 29-58 (depends on instance) |
| GKE | 10.0.0.0/14 | 110 |
| AKS | 10.244.0.0/16 | 250 |
EKS-Specific: VPC CNI
AWS EKS uses the VPC CNI plugin by default, which assigns VPC IPs directly to pods. This means pod IPs come from your VPC CIDR, consuming real subnet space.
For a 50-node EKS cluster with m5.large instances (29 pods each):
50 nodes x 29 pods = 1,450 pod IPs needed
Plus node IPs: 50
Plus headroom (20%): 300
Total: ~1,800 IPs -> /21 (2,048 IPs) minimum per AZ
Using Secondary CIDRs for Pods
To avoid exhausting your main VPC CIDR, use the EKS custom networking feature with a secondary CIDR:
VPC Primary: 10.0.0.0/16 (node ENIs, services)
VPC Secondary: 100.64.0.0/16 (pod ENIs via custom networking)
The 100.64.0.0/10 range is ideal because it does not conflict with RFC 1918 private ranges, avoiding peering issues.
Service CIDR
Do not forget the Service CIDR (default: 10.96.0.0/12 in kubeadm). This must not overlap with the pod CIDR or node network:
Node network: 10.0.0.0/16
Pod CIDR: 10.244.0.0/16
Service CIDR: 10.96.0.0/12
Use this calculator to verify none of these three ranges overlap.
Use Case
Planning the network architecture for a new Kubernetes cluster, calculating how many pod IPs are needed based on node count and pods-per-node limits, or configuring EKS custom networking with secondary VPC CIDRs.