You are designing an IP address scheme for new private Google Kubernetes Engine (GKE) clusters,...

The correct answer is D. Create privately used public IP primary and secondary subnet ranges for the clusters. Create a private GKE cluster with the following options selected: --disable-default-snat, --enable-ip-alias, and --enable-private-nodes.

This answer is based on the following facts:

Privately used public IP (PUPI) addresses are any public IP addresses not owned by Google that a customer can use privately on Google Cloud1. You can use PUPI addresses for GKE pods and services in private clusters to mitigate address exhaustion.

A private GKE cluster is a cluster that has no public IP addresses on the nodes2. You can use private clusters to isolate your workloads from the public internet and enhance security.

The --disable-default-snat option disables source network address translation (SNAT) for the cluster3. This option allows you to use PUPI addresses without conflicting with other public IP addresses on the internet.

The --enable-ip-alias option enables alias IP ranges for the cluster4. This option allows you to use separate subnet ranges for nodes, pods, and services, and to specify the size of those ranges.

The --enable-private-nodes option enables private nodes for the cluster5. This option ensures that the nodes have no public IP addresses and can only communicate with other Google Cloud resources in the same VPC network or peered networks.

The other options are not correct because:

Option A is not suitable. Creating RFC 1918 primary and secondary subnet IP ranges for the clusters does not solve the problem of address exhaustion. Re-using the secondary address range for pods across multiple private GKE clusters can cause IP conflicts and routing issues.

Option B is also not suitable. Creating RFC 1918 primary and secondary subnet IP ranges for the clusters does not solve the problem of address exhaustion. Re-using the secondary address range for services across multiple private GKE clusters can cause IP conflicts and routing issues.

Option C is not feasible. Creating privately used public IP primary and secondary subnet ranges for the clusters is a valid step, but creating a private GKE cluster with only --enable-ip-alias and --enable-private-nodes options is not enough. You also need to disable default SNAT to avoid IP conflicts with other public IP addresses on the internet.