The correct answer is to use a network ACL with a deny rule for inbound SSH traffic from 0.0.0.0/0 at the subnet level. This is the most secure and scalable approach because network ACLs are stateless and act as a firewall at the subnet boundary, blocking all port 22 traffic from the internet before it ever reaches any EC2 instance. Security groups, by contrast, are stateful and only support allow rules, making them incapable of explicitly denying traffic. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of the fundamental difference between network ACLs and security groups—specifically that NACLs can deny traffic and operate at the subnet level, while security groups are instance-level and allow-only. A common trap is assuming you can add a deny rule to a security group, but you cannot. For a quick memory tip: think of NACLs as the "bouncer at the VPC door" who can say no, while security groups are the "guest list" that only says yes.
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 Security Engineer needs to block SSH traffic (port 22) from the internet to all EC2 instances in a VPC. Which approach is the most secure and scalable?
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 to deny inbound traffic on port 22 from 0.0.0.0/0 at the subnet level.
Option A is correct because a network ACL at the subnet level blocks all traffic to port 22 from the internet before it reaches instances. Option B is wrong because you cannot deny rules in security groups; you can only allow. Option C is wrong because NACLs are stateless and require explicit outbound rules; however, the question asks to block inbound SSH, which a NACL can do. Option D is wrong because security groups are not applied at the VPC level.
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.
✗
Add a security group rule to deny inbound traffic on port 22 from 0.0.0.0/0.
Why it's wrong here
Security groups do not support deny rules; they only allow.
✓
Add a network ACL rule to deny inbound traffic on port 22 from 0.0.0.0/0 at the subnet level.
Why this is correct
Network ACLs can deny traffic and are applied at the subnet boundary.
Add a network ACL rule to allow inbound traffic on port 22 from 0.0.0.0/0 and then add a deny rule for the same traffic.
Why it's wrong here
NACL rules are evaluated in order; a deny rule would work but allowing first is unnecessary and could be confusing.
✗
Add a security group rule to block inbound traffic on port 22 from 0.0.0.0/0 at the VPC level.
Why it's wrong here
Security groups are not applied at the VPC level; they are attached to resources.
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.
Trap categories for this question
Similar concept trap
NACL rules are evaluated in order; a deny rule would work but allowing first is unnecessary and could be confusing.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 to deny inbound traffic on port 22 from 0.0.0.0/0 at the subnet level. — Option A is correct because a network ACL at the subnet level blocks all traffic to port 22 from the internet before it reaches instances. Option B is wrong because you cannot deny rules in security groups; you can only allow. Option C is wrong because NACLs are stateless and require explicit outbound rules; however, the question asks to block inbound SSH, which a NACL can do. Option D is wrong because security groups are not applied at the VPC level.
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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