The connection fails because the inbound HTTPS return traffic is denied by rule 200. This occurs because while the outbound rule correctly allows ephemeral ports for return traffic to any destination, the inbound rules do not permit the corresponding SYN-ACK packet from the internet. Specifically, inbound rule 100 only allows HTTPS traffic sourced from the subnet’s own CIDR (10.0.1.0/24), not from an external web server, and rule 200 explicitly denies all other HTTPS inbound traffic, so the return packet—with a source port of 443 and a destination ephemeral port—is dropped. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of stateful versus stateless filtering and the critical need to mirror outbound ephemeral port rules with inbound rules for return traffic. A common trap is assuming outbound rules alone suffice for return traffic, forgetting that NACLs are stateless and require explicit inbound allowances. Memory tip: “Outbound ephemeral is half the battle; inbound must mirror the source port and destination port of the reply.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 engineer runs the command above for a subnet associated with this network ACL. The subnet's CIDR is 10.0.1.0/24. An EC2 instance in the subnet attempts to initiate an HTTPS connection to a server on the internet. What is the result?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The connection fails because the inbound HTTPS return traffic is denied by rule 200.
Option C is correct. The outbound rule allows ephemeral ports for return traffic. Inbound rule 100 allows HTTPS from the subnet CIDR only, not from the internet. Rule 200 denies all other HTTPS inbound. Therefore, the outbound SYN packet is allowed by the default outbound allow (since no outbound deny), but the inbound SYN-ACK from the internet is denied by rule 200 (since it is HTTPS return traffic, source port 443, destination ephemeral port). However, the outbound rule allows traffic to any destination on ephemeral ports, but the inbound rule for return traffic must match the source port (443) and destination port (ephemeral). The inbound rules show rule 100 allows from 10.0.1.0/24 port 443, which is not the return path. Rule 200 denies all HTTPS from anywhere. So the inbound SYN-ACK is dropped. The connection fails.
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 connection fails because the outbound rule does not allow HTTPS.
Why it's wrong here
Outbound rule allows ephemeral ports; HTTPS outbound is allowed by default outbound allow (since no outbound deny).
✗
The connection succeeds because outbound traffic is allowed.
Why it's wrong here
Outbound traffic is allowed, but inbound return traffic is blocked.
✓
The connection fails because the inbound HTTPS return traffic is denied by rule 200.
Why this is correct
Return traffic (SYN-ACK) from internet has source port 443, which matches inbound deny rule 200.
The connection succeeds because inbound rule 100 allows HTTPS from the subnet.
Why it's wrong here
Rule 100 allows HTTPS from subnet CIDR as source, but return traffic comes from internet, not subnet.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The connection fails because the inbound HTTPS return traffic is denied by rule 200. — Option C is correct. The outbound rule allows ephemeral ports for return traffic. Inbound rule 100 allows HTTPS from the subnet CIDR only, not from the internet. Rule 200 denies all other HTTPS inbound. Therefore, the outbound SYN packet is allowed by the default outbound allow (since no outbound deny), but the inbound SYN-ACK from the internet is denied by rule 200 (since it is HTTPS return traffic, source port 443, destination ephemeral port). However, the outbound rule allows traffic to any destination on ephemeral ports, but the inbound rule for return traffic must match the source port (443) and destination port (ephemeral). The inbound rules show rule 100 allows from 10.0.1.0/24 port 443, which is not the return path. Rule 200 denies all HTTPS from anywhere. So the inbound SYN-ACK is dropped. The connection fails.
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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