AWS Certified Advanced Networking – Specialty (ANS-C00) — Question 370

A Network Engineer is provisioning a subnet for a load balancer that will sit in front of a fleet of application servers in a private subnet. There is limited IP space left in the VPC CIDR. The application has few users now but is expected to grow quickly to millions of users.
What design will use the LEAST amount of IP space, while allowing for this growth?

Answer options

• A. Use two /29 subnets for an Application Load Balancer in different Availability Zones.
• B. Use one /29 subnet for the Network Load Balancer. Add another VPC CIDR to the VPC to allow for future growth.
• C. Use two /28 subnets for a Network Load Balancer in different Availability Zones.
• D. Use one /28 subnet for an Application Load Balancer. Add another VPC CIDR to the VPC to allow for future growth.

Correct answer: D

Explanation

AWS subnets require a minimum size of /28 because smaller allocations like /29 leave only 3 usable IP addresses after AWS reserves 5, which is insufficient for an Application Load Balancer (ALB) to operate and scale. Utilizing a single /28 subnet minimizes the current IP consumption within the constrained primary CIDR. To facilitate future growth to millions of users, adding a secondary VPC CIDR block later provides the necessary scalability without exhausting the limited initial IP space.