To determine the appropriate subnet class for an Amazon EKS (Elastic Kubernetes Service) cluster with 5 nodes, it’s important to account for both the nodes and the additional IP addresses needed for pods and other resources. Here’s a recommended approach:

Calculation and Considerations:

1. EKS Node IP Addresses:
   • Each node will need its own IP address.
   • For 5 nodes, that’s 5 IP addresses.
2. Pod IP Addresses:
   • By default, the Amazon VPC CNI plugin assigns one IP address per pod from the node’s subnet.
   • The number of pods per node depends on your instance type and the configuration of your Kubernetes cluster.
   • For example, if you expect each node to host around 20 pods, you’ll need approximately 100 IP addresses for pods.
3. Additional Resources:
   • Include IP addresses for other resources like load balancers, services, etc.

Subnet Size Recommendation:

A /24 subnet provides 254 usable IP addresses, which is typically sufficient for a small EKS cluster with 5 nodes.

Example Calculation:

• Nodes: 5 IP addresses
• Pods: 100 IP addresses (assuming 20 pods per node)
• Additional Resources: 10 IP addresses (for services, load balancers, etc.)

Total IP Addresses Needed: 5 (nodes) + 100 (pods) + 10 (resources) = 115 IP addresses.

Recommended Subnet Size:

A /24 subnet should be sufficient for this setup:

• CIDR Notation: 192.168.0.0/24
• Total IP Addresses: 256
• Usable IP Addresses: 254

Example Configuration:

• Subnet 1: 192.168.0.0/24

Reasons to Choose a Bigger Subnet (e.g., /22 or /20):

1. Future Scalability: If you anticipate significant growth in the number of nodes or pods, a larger subnet will provide ample IP addresses for future expansion without the need to reconfigure your network.
2. Flexibility: More IP addresses give you flexibility to add additional resources such as load balancers, services, or new applications.
3. Avoiding Exhaustion: Ensuring you have a large pool of IP addresses can prevent issues related to IP address exhaustion, which can disrupt your cluster’s operations.

Example Subnet Sizes:

• /22 Subnet:
  • Total IP Addresses: 1,024
  • Usable IP Addresses: 1,022
• /20 Subnet:
  • Total IP Addresses: 4,096
  • Usable IP Addresses: 4,094

When to Consider Smaller Subnets (e.g., /24):

1. Small Deployments: If your EKS cluster is small and you do not expect significant growth, a /24 subnet might be sufficient.
2. Cost Efficiency: Smaller subnets can sometimes be more cost-effective in environments where IP address scarcity is not a concern.

For an EKS cluster with 5 nodes, I would recommend going with a /22 subnet. This gives you a healthy margin of IP addresses for your nodes, pods, and additional resources while providing room for future growth.