The correct approach is to add both inbound and outbound NACL rules denying SSH from the 10.0.0.0/8 range. This is required because NACLs are stateless, meaning they do not automatically allow return traffic; each direction must be explicitly evaluated. An inbound rule blocks the SSH request from the source IP range, but the corresponding outbound rule is necessary to block the response traffic that would otherwise be allowed by the default ephemeral port range. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of stateless versus stateful filtering, a common trap being that candidates forget to apply the outbound deny rule when blocking a specific source. A helpful memory tip is “stateless means two rules: one in, one out”—always mirror your deny rules for both directions when using NACLs.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 is configuring Network Access Control Lists (NACLs) for a VPC with multiple subnets. The engineer wants to block SSH access (port 22) from a specific IP range 10.0.0.0/8 to the entire VPC CIDR (172.16.0.0/16). What is the most effective approach?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add inbound and outbound NACL rules denying SSH from 10.0.0.0/8
Option C is correct because NACLs are stateless, so you must add inbound and outbound rules for each direction. Option A is wrong because NACLs are subnet-level, not instance-level. Option B is wrong because a single rule cannot block both directions. Option D is wrong because security groups are stateful and cannot block traffic based on source IP in the outbound rule easily.
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 inbound and outbound NACL rules denying SSH from 10.0.0.0/8
Why this is correct
NACL rules must be added for both directions due to stateless nature.
Add a security group rule to deny inbound SSH from 10.0.0.0/8
Why it's wrong here
Security groups are instance-level, not suitable for subnet-level blocking.
✗
Add an inbound NACL rule denying SSH from 10.0.0.0/8
Why it's wrong here
NACLs are stateless; you also need an outbound rule to deny the return traffic.
✗
Add an outbound security group rule denying SSH to 10.0.0.0/8
Why it's wrong here
Security groups are stateful, but they cannot filter based on destination IP in outbound rules easily.
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 Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Add inbound and outbound NACL rules denying SSH from 10.0.0.0/8 — Option C is correct because NACLs are stateless, so you must add inbound and outbound rules for each direction. Option A is wrong because NACLs are subnet-level, not instance-level. Option B is wrong because a single rule cannot block both directions. Option D is wrong because security groups are stateful and cannot block traffic based on source IP in the outbound rule easily.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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