The correct network design places both the EC2 instances and the ElastiCache Redis cluster in private subnets with tightly scoped security groups. This configuration ensures the private ElastiCache Redis cluster for ALB backend remains inaccessible from the internet while allowing the application servers to communicate with Redis over the VPC’s internal network. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of subnet isolation and security group chaining for stateful session storage—a common trap is assuming a NAT Gateway is required for private subnet communication, but within the same VPC, traffic flows natively without it. Remember that the ALB can be internet-facing in a public subnet, but the Redis cluster must never leave the private subnet boundary. A useful memory tip: “Redis stays private, ALB can greet the public, EC2 meets Redis inside the VPC.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company is running a web application on EC2 instances behind an Application Load Balancer (ALB) in a VPC. The application needs to store session state in an ElastiCache Redis cluster. The Redis cluster should not be accessible from the internet. Which network design should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Place both the EC2 instances and the Redis cluster in private subnets with appropriate security groups.
The Redis cluster should be in a private subnet to prevent internet access. The ALB is typically in a public subnet or a private subnet with an internet-facing ALB. The EC2 instances should be in a private subnet and access Redis via the private subnet. Option A is wrong because a public subnet would expose Redis. Option B is wrong because placing Redis in a public subnet exposes it. Option C is wrong because a NATGateway is not needed for private subnet communication within the VPC.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Place the Redis cluster in a public subnet and restrict access via security group.
Why it's wrong here
Public subnet implies internet exposure, even with security groups.
✗
Place the Redis cluster in a private subnet and the EC2 instances in a private subnet, with a NAT Gateway for outbound traffic.
Why it's wrong here
NATGateway is for internet access, not needed for VPC internal communication.
✗
Place the Redis cluster in a private subnet and the EC2 instances in a public subnet.
Why it's wrong here
EC2 instances should be in private subnets for security.
✓
Place both the EC2 instances and the Redis cluster in private subnets with appropriate security groups.
Why this is correct
Both in private subnets, security groups allow traffic from EC2 to Redis.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Place both the EC2 instances and the Redis cluster in private subnets with appropriate security groups. — The Redis cluster should be in a private subnet to prevent internet access. The ALB is typically in a public subnet or a private subnet with an internet-facing ALB. The EC2 instances should be in a private subnet and access Redis via the private subnet. Option A is wrong because a public subnet would expose Redis. Option B is wrong because placing Redis in a public subnet exposes it. Option C is wrong because a NAT Gateway is not needed for private subnet communication within the VPC.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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