The answer is Option A: Inbound allow TCP 443 from 0.0.0.0/0, and Outbound allow TCP 1024-65535 to 0.0.0.0/0 plus TCP 443 to 0.0.0.0/0. This is the minimum set because Network ACLs are stateless, meaning return traffic for an inbound HTTPS request is not automatically allowed; you must explicitly permit ephemeral ports (1024-65535) outbound for the web server to send responses back to clients. Additionally, the outbound rule for TCP 443 to 0.0.0.0/0 covers the server’s own outbound connections for software updates. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of stateless vs. stateful filtering—a common trap is forgetting that NACLs require separate rules for return traffic, unlike Security Groups. A frequent distractor is allowing all outbound traffic (Option C), which is not minimal, or restricting outbound to only a specific update server (Option D), which breaks the general internet access requirement. Memory tip: Stateless means “state-less” rules—you must write both the request and the reply path.
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 a Network ACL for a public subnet that hosts a web server. The web server must accept HTTPS (TCP 443) traffic from the internet and respond. It must also be able to initiate outbound connections to the internet for software updates (HTTPS). What is the MINIMUM set of rules required for the inbound and outbound Network ACL?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inbound: allow TCP 443 from 0.0.0.0/0; Outbound: allow TCP 1024-65535 to 0.0.0.0/0 (for return traffic) and allow TCP 443 to 0.0.0.0/0 (for updates).
Option A is correct because inbound allows HTTPS from anywhere (0.0.0.0/0) and outbound allows ephemeral ports for return traffic and destination 0.0.0.0/0 for the outbound updates. Option B is wrong because it allows all inbound traffic. Option C is wrong because it allows all outbound traffic, which is not minimal but acceptable, but it also allows inbound SSH, which is not needed. Option D is wrong because it restricts outbound to only the update server, which may be too restrictive.
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.
✗
Inbound: allow TCP 443 from 0.0.0.0/0 and TCP 22 from a management IP; Outbound: allow all traffic to 0.0.0.0/0.
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 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: Inbound: allow TCP 443 from 0.0.0.0/0; Outbound: allow TCP 1024-65535 to 0.0.0.0/0 (for return traffic) and allow TCP 443 to 0.0.0.0/0 (for updates). — Option A is correct because inbound allows HTTPS from anywhere (0.0.0.0/0) and outbound allows ephemeral ports for return traffic and destination 0.0.0.0/0 for the outbound updates. Option B is wrong because it allows all inbound traffic. Option C is wrong because it allows all outbound traffic, which is not minimal but acceptable, but it also allows inbound SSH, which is not needed. Option D is wrong because it restricts outbound to only the update server, which may be too restrictive.
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.
Are there clue words in this question I should notice?
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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