The correct answer is to place all ECS services in private subnets and use AWS Cloud Map for service discovery. This configuration ensures that microservices running on ECS Fargate can resolve and communicate with each other via internal DNS names without any exposure to the internet, as private subnets have no route to an internet gateway and Cloud Map registers service instances in a private namespace. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of secure inter-service communication within a VPC, often appearing as a distractor against options like AWS PrivateLink (which is for accessing external services, not internal service-to-service discovery) or VPC peering (which connects VPCs, not services). A common trap is assuming that service discovery requires public endpoints or a load balancer, but Cloud Map’s private DNS namespace handles this natively. Memory tip: think “Private + Cloud Map = Internal handshake, no internet escape.”
SAP-C02 Design for New Solutions Practice Question
This SAP-C02 practice question tests your understanding of design for new solutions. 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 designing a microservices architecture on Amazon ECS with Fargate. They want to ensure that services can communicate with each other but are isolated from the internet. What is the MOST secure way to achieve this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Place all ECS services in private subnets and use AWS Cloud Map for service discovery.
Option A is correct because placing services in private subnets and using ECS service discovery with AWS Cloud Map allows internal DNS resolution without internet exposure. Option B is wrong because AWS PrivateLink is used for accessing services over endpoints, not service-to-service. Option C is wrong because VPC peering is for connecting VPCs, not for service discovery. Option D is wrong because placing in public subnets exposes services to the internet.
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.
✗
Use VPC peering to connect the subnets of each service.
Why it's wrong here
VPC peering connects VPCs, but does not provide service discovery.
✗
Use AWS PrivateLink to create VPC endpoints for each service.
Why it's wrong here
PrivateLink is used to access services across VPCs, not for internal service discovery.
✗
Place services in public subnets and use security groups to restrict inbound traffic.
Why it's wrong here
Public subnets expose services to the internet, increasing risk.
✓
Place all ECS services in private subnets and use AWS Cloud Map for service discovery.
Why this is correct
Private subnets ensure no internet exposure; Cloud Map provides DNS-based service discovery.
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 SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Design for New Solutions — This question tests Design for New Solutions — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Place all ECS services in private subnets and use AWS Cloud Map for service discovery. — Option A is correct because placing services in private subnets and using ECS service discovery with AWS Cloud Map allows internal DNS resolution without internet exposure. Option B is wrong because AWS PrivateLink is used for accessing services over endpoints, not service-to-service. Option C is wrong because VPC peering is for connecting VPCs, not for service discovery. Option D is wrong because placing in public subnets exposes services to the internet.
What should I do if I get this SAP-C02 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 SAP-C02 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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Question Discussion
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