The answer is to add a network ACL rule on the public subnet to deny inbound traffic from the specific IP on port 22. This is correct because a network ACL operates at the subnet layer as a stateless firewall, allowing you to explicitly deny traffic from a particular source IP and port without affecting any other traffic, unlike security groups which only support allow rules and cannot block a single IP. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the fundamental difference between stateful security groups and stateless network ACLs, often appearing as a trap where candidates mistakenly try to use a security group deny rule or overcomplicate with WAF. A common memory tip is "ACLs are for blocking, SGs are for allowing"—remember that security groups cannot say "no" to a specific IP, but network ACLs can.
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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 public subnet and a private subnet. The public subnet hosts a NAT instance (Amazon Linux) that provides internet access to instances in the private subnet. The security team notices that the NAT instance is receiving high inbound traffic on port 22 from an external IP address. The team wants to block this traffic at the network layer without affecting other traffic. What is the most effective solution?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a network ACL rule on the public subnet to deny inbound traffic from the specific IP on port 22.
Option D is correct because a network ACL on the public subnet can explicitly deny inbound SSH from the specific IP address while allowing all other traffic. Option A is wrong because modifying the security group of the NAT instance to deny the IP would require a deny rule, but security groups only support allow rules; blocking SSH from all sources would prevent legitimate access. Option B is wrong because moving the NAT instance to a private subnet would break internet connectivity. Option C is wrong because AWS WAF is for web application layer (HTTP/HTTPS), not SSH.
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.
✗
Move the NAT instance to a private subnet and use a NAT gateway instead.
Why it's wrong here
This would not block the traffic and changes architecture.
✗
Modify the security group attached to the NAT instance to block inbound SSH from the specific IP.
Why it's wrong here
Security groups cannot have explicit deny rules; removing the SSH rule would block all SSH.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Add a network ACL rule on the public subnet to deny inbound traffic from the specific IP on port 22. — Option D is correct because a network ACL on the public subnet can explicitly deny inbound SSH from the specific IP address while allowing all other traffic. Option A is wrong because modifying the security group of the NAT instance to deny the IP would require a deny rule, but security groups only support allow rules; blocking SSH from all sources would prevent legitimate access. Option B is wrong because moving the NAT instance to a private subnet would break internet connectivity. Option C is wrong because AWS WAF is for web application layer (HTTP/HTTPS), not SSH.
What should I do if I get this SCS-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 SCS-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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