AWS Certified Advanced Networking – Specialty (ANS-C00) — Question 54

An organization launched an IPv6-only web portal to support IPv6-native mobile clients. Front-end instances launch in an Amazon VPC associated with an appropriate IPv6 CIDR. The VPC IPv4 CIDR is fully utilized. A single subnet exists in each of two Availability Zones with appropriately configured IPv6 CIDR associations. Auto Scaling is properly configured, and no Elastic Load Balancing is used.
Customers say the service is unavailable during peak load times. The network engineer attempts to launch an instance manually and receives the following message: `There are not enough free addresses in subnet `˜subnet-12345678' to satisfy the requested number of instances.`
What action will resolve the availability problem?

Answer options

• A. Create a new subnet using a VPC secondary IPv6 CIDR, and associate an IPv6 CIDR. Include the new subnet in the Auto Scaling group.
• B. Create a new subnet using a VPC secondary IPv4 CIDR, and associate an IPv6 CIDR. Include the new subnet in the Auto Scaling group.
• C. Resize the IPv6 CIDR on each of the existing subnets. Modify the Auto Scaling group maximum number of instances.
• D. Add a secondary IPv4 CIDR to the Amazon VPC. Assign secondary IPv4 address space to each of the existing subnets.

Correct answer: B

Explanation

Option B is correct because creating a new subnet with a secondary IPv4 CIDR allows for the allocation of additional IP addresses, which can be paired with an IPv6 CIDR to accommodate more instances. The other options do not address the IPv4 address shortage directly, and simply resizing or modifying existing CIDRs will not resolve the immediate issue of address availability.