An administrator creates a new VPC containing one public subnet and one private subnet. The administrator successfully launches 11 EC2 instances inside the private subnet. When attempting to start a 12th instance in the same subnet, the launch fails with an error indicating there are no available IP addresses. What should the administrator do to accommodate additional EC2 instances in that subnet?

The correct answer is to create an additional private subnet and place the new EC2 instances there. Subnets have a fixed CIDR block that determines the number of available IP addresses. Once a subnet is created, its CIDR block cannot be modified. Launching 11 instances in a subnet that only supports 11 usable IPs (e.g., a /28 CIDR block, which provides 16 IPs, but 5 are reserved by AWS) means the subnet is full. Creating a new subnet provides a new range of IP addresses for additional instances. Modifying the private subnet’s CIDR is not possible after creation. Changing the private subnet to span a second Availability Zone is incorrect because subnets are tied to a single Availability Zone. Attaching more Elastic IP addresses is irrelevant as Elastic IPs are for public internet access and do not increase the number of private IP addresses available within a subnet.