An organization launched an IPv6-only web portal to support IPv6-native mobile clients. Front-end instances launch in an Amazon VPC associated with…

Even in an IPv6-enabled VPC and subnet, every EC2 instance requires a primary private IPv4 address for its primary network interface (ENI). The error message "not enough free addresses in subnet" confirms the issue is IPv4 address exhaustion within the existing subnets. The question states the VPC's primary IPv4 CIDR is fully utilized. The correct resolution is to add a secondary IPv4 CIDR block to the VPC, create a new subnet from this new address space, and then configure the Auto Scaling group to launch new instances into this new subnet, which has available IPv4 addresses.

A. Creating a new subnet with only an IPv6 CIDR does not solve the underlying requirement for a primary private IPv4 address for each new instance.
C. Resizing the IPv6 CIDR is irrelevant as the problem is IPv4 address exhaustion, not IPv6. Instances still require an IPv4 address to launch.
D. You cannot assign a secondary IPv4 CIDR block to an existing subnet. New subnets must be created from the new VPC secondary CIDR.

1. AWS Documentation, VPC User Guide, "VPC and subnet sizing for IPv4". Under the "Subnet sizing" section, it states, "Every instance that you launch into a subnet requires an IP address from that subnet's CIDR block."

2. AWS Documentation, EC2 User Guide, "Amazon EC2 instance IP addressing". Under "Private IPv4 addresses and internal DNS hostnames", it clarifies, "When you launch an instance into a VPC, a primary private IPv4 address from the IPv4 address range of the subnet is assigned to the primary network interface."

3. AWS Documentation, VPC User Guide, "Add an IPv4 CIDR block to your VPC". This guide details the process: "After you add a CIDR block to your VPC, you can create new subnets in that CIDR block."