The Jeff’s Note (Contextual Hook) #
Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Site Reliability Engineer (SRE).
For SOA-C02 candidates, the confusion often lies in thinking more IP addresses can be assigned within a fixed subnet mask. In production, this is about knowing exactly how subnet CIDR blocks constrain your usable IP space and how to scale subnets properly. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
TitanTech Solutions, a rapidly growing IoT startup, set up a new AWS VPC with one public subnet and one private subnet. The DevOps team has successfully launched 11 Amazon EC2 instances inside the private subnet that supports their internal application servers. When they try to launch an additional EC2 instance in the same private subnet, the launch fails with an error saying there are not enough free IP addresses available.
The Requirement: #
What must the DevOps team do to deploy more EC2 instances within the private subnet without encountering IP exhaustion errors?
The Options #
- A) Modify the private subnet’s CIDR block to use a /27 mask instead of the current setting.
- B) Configure the private subnet to extend across a second Availability Zone.
- C) Assign additional Elastic IP addresses to the private subnet.
- D) Create a new private subnet with sufficient IP addresses to host more EC2 instances.
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Correct Answer #
D
Quick Insight: The SysOps Imperative #
When managing VPC subnet IP exhaustion, understanding subnet sizing and CIDR block immutability is critical.
You cannot expand a subnet’s IP range after creation, nor do Elastic IPs increase subnet IP capacity.
Creating new subnets in the VPC with appropriately sized CIDR blocks is the best scalable path.
Content Locked: The Expert Analysis #
You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?
The Expert’s Analysis #
Correct Answer #
Option D
The Winning Logic #
Each subnet in a VPC has a fixed CIDR block that defines the range of IP addresses available. AWS reserves five IP addresses per subnet, reducing the usable IP count slightly. Once a subnet is created, you cannot modify its CIDR block or enlarge it. Therefore, if you run out of available IP addresses to assign to new EC2 instances, the proper solution is to create a new subnet with a larger or additional CIDR block that can accommodate more instances. This is a common and standard practice when scaling infrastructure.
The Trap (Distractor Analysis) #
-
Why not A?
You cannot edit or change a subnet’s CIDR block after creation. Changing from a /28 (for example) to a /27 is not possible. You would have to delete and create a new subnet, which risks downtime and loss of resources assigned to the original subnet. -
Why not B?
Extending a subnet across Availability Zones is not supported. Each subnet resides in exactly one AZ. Creating subnets in multiple AZs is a good practice for high availability, but that requires creating separate subnets, not extending the existing one. -
Why not C?
Elastic IP addresses are static public IPv4 addresses assigned to instances or NAT gateways, not private subnet IP ranges. Assigning additional Elastic IPs does not increase the number of private IPs available inside the subnet.
The Technical Blueprint #
# Check number of available IPs in subnet (replace subnet-id)
aws ec2 describe-subnets --subnet-ids subnet-xxxxxxxx --query 'Subnets[0].AvailableIpAddressCount'
# Create a new subnet example (CIDR block must not overlap existing ones)
aws ec2 create-subnet --vpc-id vpc-xxxxxxxx --cidr-block 10.0.2.0/24 --availability-zone us-east-1b
The Comparative Analysis #
| Option | Operational Overhead | Automation Level | Impact on Deployment |
|---|---|---|---|
| A | High (requires subnet recreation) | Low | Impossible to modify existing subnet CIDR range |
| B | Medium (creating subnet in different AZ needed) | Medium | Misunderstood concept; no subnet extension across AZs |
| C | Low | N/A | Elastic IPs do not affect subnet IP range |
| D | Low to medium (add new subnet) | High | Correct and scalable method to increase IP capacity |
Real-World Application (Practitioner Insight) #
Exam Rule #
For the exam, always remember: “You cannot resize a subnet’s CIDR block once created.” Always plan subnet CIDR blocks with adequate padding or add new subnets to increase IP availability.
Real World #
In practice, many companies adopt a multi-subnet approach from the start, designing CIDR blocks to accommodate growth and maximize multi-AZ fault tolerance.
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the AWS SOA-C02 exam.