The answer is no, SSH traffic from that IP range will not reach the instance because the network ACL denies all inbound traffic. This is correct because network ACLs are stateless and operate at the subnet level, meaning they evaluate traffic before any security group rules are applied; since the ACL explicitly blocks all inbound traffic, the packet is dropped before the security group, which allows SSH, ever gets a chance to inspect it. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the layered defense model and the evaluation order of VPC security controls—a common trap is assuming a permissive security group can override a restrictive network ACL. Remember the memory tip: ACLs are the subnet bouncer that checks ID first, while security groups are the instance doorman who only sees guests who already got past the bouncer.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 multiple subnets. The network ACL for a subnet is configured to deny all inbound traffic. A security group attached to an EC2 instance in that subnet allows SSH from a specific IP range. Will SSH traffic from that IP range reach the instance?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
No, because the network ACL denies all inbound traffic.
Option B is correct because network ACLs are stateless and deny all inbound traffic, so even though the security group allows SSH, the network ACL will block the traffic before it reaches the instance. Option A is wrong because security groups are stateful but are evaluated after network ACLs. Option C is wrong because the network ACL operates at the subnet level. Option D is wrong because the order does not matter for stateless ACLs.
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.
✗
Yes, because the security group allows SSH.
Why it's wrong here
The security group is evaluated after the network ACL; the network ACL blocks the traffic first.
✓
No, because the network ACL denies all inbound traffic.
Why this is correct
The network ACL is stateless and denies all inbound traffic, so the traffic is blocked.
Yes, because the security group overrides the network ACL.
Why it's wrong here
Security groups do not override network ACLs; both must allow traffic.
✗
It depends on the rule number order in the network ACL.
Why it's wrong here
If all inbound traffic is denied, the rule number does not matter.
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 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 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: No, because the network ACL denies all inbound traffic. — Option B is correct because network ACLs are stateless and deny all inbound traffic, so even though the security group allows SSH, the network ACL will block the traffic before it reaches the instance. Option A is wrong because security groups are stateful but are evaluated after network ACLs. Option C is wrong because the network ACL operates at the subnet level. Option D is wrong because the order does not matter for stateless ACLs.
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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