The answer is that the NACL’s deny all rule (rule 400) overrides the allow rules, blocking HTTP traffic despite the security group permitting it. This happens because Network ACLs are stateless and evaluate rules in ascending order by rule number; rule 400, with a higher number, is evaluated after rules 100 and 300, and since it denies all inbound traffic, it supersedes any earlier allows. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of NACL rule priority and the critical difference between stateful security groups and stateless NACLs—a common trap is assuming a lower-numbered allow rule will prevail. Remember that in a stateless NACL, the last matching rule wins, so a broad deny with a high rule number will always block traffic, even if earlier rules permit it. A useful memory tip: “Last rule wins in a stateless ACL—deny all at the end kills all.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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.
Network Topology
An EC2 instance in the subnet associated with the network ACL above cannot receive HTTP traffic (port 80) from the internet. The instance has a security group allowing HTTP inbound. What is the cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The NACL has a deny all rule (rule 400) that blocks all inbound traffic, overriding the allow rules
Option C is correct. Network ACLs are stateless and rules are evaluated in order by rule number. Rule 400 denies all inbound traffic, and since it has a higher rule number than rules 100 and 300, it is evaluated later and overrides the allows. Rule 400 denies all inbound traffic, so HTTP (port 80) is denied. Option A is wrong because rule 300 allows port 80 inbound from 0.0.0.0/0, so HTTP is allowed by that rule, but rule 400 denies all. Option B is wrong because the outbound rule allows all traffic. Option D is wrong because the security group allows HTTP inbound but the NACL blocks it.
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 NACL allows HTTP inbound on rule 300, but the outbound rule does not allow the return traffic
Why it's wrong here
Outbound allows all traffic, so return traffic is allowed.
✗
The NACL inbound rule for HTTP is using the wrong protocol (6 instead of 17)
The stem states the security group allows HTTP inbound.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The NACL has a deny all rule (rule 400) that blocks all inbound traffic, overriding the allow rules — Option C is correct. Network ACLs are stateless and rules are evaluated in order by rule number. Rule 400 denies all inbound traffic, and since it has a higher rule number than rules 100 and 300, it is evaluated later and overrides the allows. Rule 400 denies all inbound traffic, so HTTP (port 80) is denied. Option A is wrong because rule 300 allows port 80 inbound from 0.0.0.0/0, so HTTP is allowed by that rule, but rule 400 denies all. Option B is wrong because the outbound rule allows all traffic. Option D is wrong because the security group allows HTTP inbound but the NACL blocks it.
What should I do if I get this SOA-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 SOA-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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