The answer is that the RDS instance’s security group lacks an inbound rule allowing traffic from the web tier’s security group. This is because security groups are stateful and enforce isolation by explicitly permitting traffic only from specified source security groups or CIDR blocks. In a multi-tier architecture with security group isolation, each tier’s security group acts as a virtual firewall, and the database’s inbound rule for TCP 3306 is scoped solely to the application tier’s security group (sg-app), so the web tier’s instances—belonging to a different security group—are implicitly denied. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of security group behavior versus network ACLs and routing, often appearing as a trap where candidates mistakenly blame NACLs or route tables. A common memory tip is “security groups are identity-based, not path-based”—they care about who the source is, not how traffic arrives.
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 has a multi-tier application deployed in a VPC. The web tier consists of an internet-facing Application Load Balancer (ALB) in public subnets, and EC2 instances in private subnets. The application tier runs on EC2 instances in separate private subnets, and the database tier uses an Amazon RDS for MySQL instance in private subnets. The application tier needs to connect to the database on port 3306. The security group for the RDS instance (sg-database) has an inbound rule allowing TCP 3306 from the security group of the application tier (sg-app). The application tier instances can connect to the database, but the web tier instances cannot. The web tier instances should not have direct database access. What is the most likely reason for the web tier's inability to connect to the database?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group of the RDS instance does not have an inbound rule allowing traffic from the web tier's security group.
The security group rule on sg-database allows traffic from sg-app. Since the web tier instances are in a different security group, they are not allowed. Option A is wrong because the rule is from sg-app, not from the web tier's security group. Option B is wrong because NACLs are stateless and if they were blocking, the application tier would also be affected. Option C is wrong because the database is in a private subnet and does not need a NATGateway. Option D is wrong because the route table is not relevant for security group rules.
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.
✗
The network ACL for the database subnet is blocking inbound traffic from the web tier subnets.
Why it's wrong here
NACLs are stateless; if they blocked, the application tier would also be blocked.
✓
The security group of the RDS instance does not have an inbound rule allowing traffic from the web tier's security group.
Why this is correct
The rule only allows traffic from sg-app, not from the web tier's security group.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route table for the web tier subnets does not have a route to the database subnets.
Why it's wrong here
Within a VPC, routes are implicit; no additional route needed.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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: The security group of the RDS instance does not have an inbound rule allowing traffic from the web tier's security group. — The security group rule on sg-database allows traffic from sg-app. Since the web tier instances are in a different security group, they are not allowed. Option A is wrong because the rule is from sg-app, not from the web tier's security group. Option B is wrong because NACLs are stateless and if they were blocking, the application tier would also be affected. Option C is wrong because the database is in a private subnet and does not need a NAT Gateway. Option D is wrong because the route table is not relevant for security group rules.
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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is running a two-tier application with a web tier and a database tier. The web tier must be accessible from the internet, but the database tier should only be accessible from the web tier. Which architecture should they use?
hard
✓ A.Web tier in public subnet, database tier in private subnet with a security group allowing only web tier.
B.Both tiers in public subnets with security groups restricting database access.
C.Both tiers in private subnets with a NAT Gateway for internet access.
D.Web tier in private subnet, database tier in public subnet with a security group.
Why A: The correct answer is C because placing the web tier in a public subnet with an Internet Gateway and the database tier in a private subnet with a security group that allows traffic only from the web tier provides the required security. Option A (both in public) exposes the database. Option B (both in private) prevents internet access. Option D (database in public) is insecure.
Variation 2. A company is deploying a multi-tier application on AWS and needs to ensure that traffic between the web tier and the application tier does not traverse the internet. Both tiers are deployed in the same VPC but in different subnets. What is the MOST secure way to meet this requirement?
easy
A.Use an Internet Gateway to route traffic between subnets
B.Use VPC Peering between the subnets
C.Use a NAT Gateway in each subnet for inter-subnet communication
✓ D.Use route tables and security groups to allow traffic within the VPC
Why D: Option C is correct because VPC Peering is used for connectivity between different VPCs, not within the same VPC. Option A is wrong because internet traffic would still traverse the internet if using an Internet Gateway. Option B is wrong because NAT Gateways are for outbound internet access, not for private connectivity. Option D is wrong because VPC Endpoints are for accessing AWS services privately, not for traffic between tiers within a VPC.
Variation 3. A company is deploying a multi-tier web application on AWS. The web tier runs on EC2 instances behind an Application Load Balancer (ALB), and the application tier runs on EC2 instances that connect to an RDS MySQL Multi-AZ DB instance. The application tier must be isolated from the internet and only accessible from the web tier. Which network implementation meets these requirements with the LEAST administrative overhead?
medium
✓ A.Place the web tier in a public subnet and the application tier in a private subnet, and configure security groups to allow inbound traffic to the application tier only from the web tier's security group.
B.Create two VPCs: one for the web tier with a public subnet, and one for the application tier with a private subnet, and connect them using VPC peering.
C.Place the web tier in a public subnet and the application tier in a private subnet with a NAT gateway for outbound access, and use security groups to allow traffic from the web tier.
D.Place both tiers in the same public subnet, and use a network ACL to restrict traffic from the web tier to the application tier.
Why A: Option C is correct because placing the web tier in a public subnet and the application tier in a private subnet with security groups allowing traffic only from the web tier is the standard design for multi-tier web applications with minimal overhead. Option A is wrong because using a network ACL alone would not isolate the application tier from the internet; a private subnet is needed. Option B is wrong because a NAT gateway is unnecessary for outbound access from the application tier (not required) and adds cost. Option D is wrong because using a different VPC and VPC peering adds complexity.
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