The correct answer is to place the RDS instance in two private subnets that are in different Availability Zones. This is because a Multi-AZ deployment requires the primary and standby database instances to reside in separate subnets across distinct AZs to ensure automatic failover and data durability. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of subnet placement for high availability, often disguised as a trick where candidates mistakenly choose a single private subnet or public subnets. A common trap is assuming any private subnet works, but the key is that the two subnets must be in different AZs—simply being private is insufficient. Memory tip: think "two private, two AZs" to avoid the single-subnet pitfall.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 has a VPC with a CIDR of 10.0.0.0/16. It has six subnets: three public (10.0.1.0/24, 10.0.2.0/24, 10.0.3.0/24) and three private (10.0.4.0/24, 10.0.5.0/24, 10.0.6.0/24). The company wants to launch an RDS instance in a private subnet. Which subnet should the RDS instance be placed in to maximize high availability and follow best practices?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Place the RDS instance in two private subnets that are in different Availability Zones.
RDS requires two subnets in different AZs for Multi-AZ deployments. The private subnets 10.0.4.0/24, 10.0.5.0/24, and 10.0.6.0/24 are in three different AZs (assuming each subnet is in a different AZ). Option A is correct because it indicates using two private subnets in different AZs. Option B is incorrect because public subnets expose the database. Option C is incorrect because a single subnet does not provide high availability. Option D is incorrect because RDS does not require all six subnets.
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 RDS instance in a public subnet and use security groups to restrict access.
Why it's wrong here
RDS should not be in a public subnet; it should be private.
✗
Place the RDS instance in all three private subnets to maximize availability.
Why it's wrong here
RDS requires exactly two subnets for Multi-AZ.
✓
Place the RDS instance in two private subnets that are in different Availability Zones.
Why this is correct
This allows Multi-AZ deployment for high availability.
Clue confirmation
The clue word "best" in the question point toward this answer.
Place the RDS instance in a single private subnet with a large CIDR for future scaling.
Why it's wrong here
Single subnet does not support Multi-AZ.
Common exam traps
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Place the RDS instance in two private subnets that are in different Availability Zones. — RDS requires two subnets in different AZs for Multi-AZ deployments. The private subnets 10.0.4.0/24, 10.0.5.0/24, and 10.0.6.0/24 are in three different AZs (assuming each subnet is in a different AZ). Option A is correct because it indicates using two private subnets in different AZs. Option B is incorrect because public subnets expose the database. Option C is incorrect because a single subnet does not provide high availability. Option D is incorrect because RDS does not require all six subnets.
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.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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