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Network Management and Operations →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is the network ACL allowing all inbound traffic, which lets packets from the internet reach the database subnet before the security group can block them. This occurs because a network ACL is stateless and evaluates traffic at the subnet boundary first, regardless of the security group’s rules. In this scenario, the NACL permits all inbound traffic on all ports, so SYN packets from an unknown IP are logged by VPC Flow Logs even though the security group later denies them. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of the order of evaluation: NACLs operate at the subnet level and are stateless, while security groups are stateful and operate at the instance level. A common trap is assuming security groups block traffic before NACLs, but in reality, traffic must pass the NACL first. Remember the memory tip: “NACL is the bouncer at the subnet door; the security group is the guard at the instance room.”

ANS-C01 Network Management and Operations Practice Question

This ANS-C01 practice question tests your understanding of network management and operations. 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 172.16.0.0/16. The VPC contains an Amazon RDS for MySQL database in a private subnet. The database is accessed by EC2 instances in the same VPC and by on-premises servers via a Site-to-Site VPN. The network team recently enabled VPC Flow Logs and noticed that the database is receiving a high number of SYN packets from an IP address that is not part of the VPC or on-premises network. The security group for the database only allows inbound traffic on port 3306 from the EC2 instances' security group and the on-premises CIDR (10.0.0.0/8). The network ACL for the database subnet allows inbound and outbound traffic on all ports from all sources. What is the most likely cause of the unexpected traffic?

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.

Question 1hardmultiple choice

Study the full ACL explanation →

A
The security group for the database has an inbound rule that allows traffic from 0.0.0.0/0.
Why wrong: The security group only allows traffic from specific sources.
B
The RDS database has a public endpoint that is accessible from the internet.
Why wrong: The database is in a private subnet and not publicly accessible.
C
The Site-to-Site VPN is misconfigured and routing internet traffic into the VPC.
Why wrong: VPN routes only on-premises traffic.
D
The network ACL for the database subnet allows all inbound traffic, so packets from the internet reach the database subnet's network ACL before being evaluated by the security group.
NACL is stateless and allows all traffic, so packets enter the subnet and are then evaluated by the security group, which drops them.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The network ACL for the database subnet allows all inbound traffic, so packets from the internet reach the database subnet's network ACL before being evaluated by the security group.

Option B is correct because a network ACL is stateless and allows traffic from any source, including the internet, because it allows all inbound traffic. Even though the security group blocks the traffic, the NACL allows it, so the packets reach the NACL and are logged. Option A is wrong because the security group is blocking the traffic, not allowing it. Option C is wrong because RDS is not publicly accessible. Option D is wrong because the VPN is only for on-premises traffic; the IP is not from on-premises.

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 security group for the database has an inbound rule that allows traffic from 0.0.0.0/0.
Why it's wrong here
The security group only allows traffic from specific sources.
✗
The RDS database has a public endpoint that is accessible from the internet.
Why it's wrong here
The database is in a private subnet and not publicly accessible.
✗
The Site-to-Site VPN is misconfigured and routing internet traffic into the VPC.
Why it's wrong here
VPN routes only on-premises traffic.
✓
The network ACL for the database subnet allows all inbound traffic, so packets from the internet reach the database subnet's network ACL before being evaluated by the security group.
Why this is correct
NACL is stateless and allows all traffic, so packets enter the subnet and are then evaluated by the security group, which drops them.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The network ACL for the database subnet allows all inbound traffic, so packets from the internet reach the database subnet's network ACL before being evaluated by the security group. — Option B is correct because a network ACL is stateless and allows traffic from any source, including the internet, because it allows all inbound traffic. Even though the security group blocks the traffic, the NACL allows it, so the packets reach the NACL and are logged. Option A is wrong because the security group is blocking the traffic, not allowing it. Option C is wrong because RDS is not publicly accessible. Option D is wrong because the VPN is only for on-premises traffic; the IP is not from on-premises.

What should I do if I get this ANS-C01 question wrong?

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.

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