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

The error message "There are not enough free addresses in subnet..." indicates the exhaustion of private IPv4 addresses. In Amazon VPC, unless using Nitro-based instances in a specifically configured IPv6-only subnet, every EC2 instance requires a primary private IPv4 address from its subnet's CIDR range upon launch. The scenario states the VPC's primary IPv4 CIDR is fully utilized, which is the root cause of the Auto Scaling failure.

The correct resolution is to add a secondary IPv4 CIDR block to the VPC. From this new address space, a new subnet can be created. This new subnet, which will have available IPv4 addresses, must then be added to the Auto Scaling group's configuration, allowing it to successfully launch new instances to meet demand.

A. This is incorrect because the problem is a lack of IPv4 addresses, not IPv6. Adding more IPv6 space does not resolve the IPv4 exhaustion issue.

C. This is incorrect because you cannot resize the IPv6 CIDR of an existing AWS subnet (it is fixed at /64), and this action would not address the underlying IPv4 address shortage.

D. This is incorrect because while adding a secondary IPv4 CIDR to the VPC is the right first step, you cannot assign a secondary IPv4 range to an existing subnet. A new subnet must be created.

1. AWS Documentation - VPCs and subnets - Amazon Virtual Private Cloud User Guide: "If you launch an instance into a default VPC without specifying a private IPv4 address, we choose an IP address for you from the IPv4 address range of the subnet... Every instance that you launch into a VPC has a primary private IPv4 address..." This confirms the requirement for a private IPv4 address. (Source: Amazon VPC User Guide, Section: "VPC and subnet basics").

2. AWS Documentation - Add an IPv4 CIDR block to your VPC - Amazon Virtual Private Cloud User Guide: "You can associate secondary IPv4 CIDR blocks with your VPC... After you've associated a CIDR block with your VPC, you can add a subnet from that CIDR block." This outlines the correct procedure of adding a secondary CIDR and then creating a new subnet from it. (Source: Amazon VPC User Guide, Section: "Your VPC and subnets", Subsection: "Add an IPv4 CIDR block to your VPC").

3. AWS Documentation - Subnet sizing for IPv4 - Amazon Virtual Private Cloud User Guide: "The IPv4 CIDR block of a subnet cannot be changed after you create it." This confirms that option D, which suggests modifying an existing subnet, is not a valid action. (Source: Amazon VPC User Guide, Section: "Your VPC and subnets", Subsection: "VPC and subnet sizing for IPv4").

4. AWS Documentation - IPv6-only subnets - Amazon Virtual Private Cloud User Guide: "Instances launched in an IPv6-only subnet receive an IPv6 address but not an IPv4 address. Instances that you launch in an IPv6-only subnet must be built on the AWS Nitro System." The generic "not enough free addresses" error strongly implies the instances are not configured for this specific IPv6-only mode, thus requiring an IPv4 address. (Source: Amazon VPC User Guide, Section: "IP addressing in your VPC", Subsection: "IPv6-only subnets").