Quick Answer
The correct inbound Network ACL rule for HTTPS traffic to an ALB in a public subnet is to allow TCP port 443 from 0.0.0.0/0. This is because HTTPS traffic relies exclusively on TCP port 443 for secure web communication, and the ALB must accept this traffic from any internet source to function as a load balancer. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of stateless Network ACL rules versus stateful security groups, and it often appears as a trap where candidates confuse inbound ephemeral ports with outbound rules or mistakenly allow UDP for HTTPS. A common mistake is selecting an overly permissive rule like allowing all traffic, which violates least privilege, or choosing ephemeral ports, which are only needed for outbound responses. Remember the key distinction: inbound rules for ALBs must specify the listener port (443) from 0.0.0.0/0, while ephemeral ports are automatically handled by the ALB’s security group for return traffic. Memory tip: HTTPS = TCP 443 from anywhere, and never use UDP for web traffic.
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 designing a network ACL for a public subnet containing an Application Load Balancer. The subnet must allow inbound HTTPS traffic from the internet and outbound traffic to the internet for patches. Which inbound rule should be added?
Allow TCP port 1024-65535 from 0.0.0.0/0
Why wrong: Ephemeral ports are for return traffic, not inbound.
Allow UDP port 443 from 0.0.0.0/0
Why wrong: HTTPS uses TCP, not UDP.
Allow all traffic from 0.0.0.0/0
Why wrong: Overly permissive; should restrict to needed ports.
Allow TCP port 443 from 0.0.0.0/0
HTTPS uses TCP port 443.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Allow TCP port 443 from 0.0.0.0/0
Option B is correct because HTTPS uses TCP port 443. Option A is wrong because HTTPS does not use UDP. Option C is wrong because the ALB should not allow all inbound traffic. Option D is wrong because ephemeral ports are for outbound, not inbound.
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
For each option: why learners choose it and why it is or isn't the right answer here.
Allow TCP port 1024-65535 from 0.0.0.0/0
Why it's wrong here
Ephemeral ports are for return traffic, not inbound.
Allow UDP port 443 from 0.0.0.0/0
Allow all traffic from 0.0.0.0/0
Why it's wrong here
Overly permissive; should restrict to needed ports.
Allow TCP port 443 from 0.0.0.0/0
Common exam traps
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
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.
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
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
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.
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FAQ
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
The correct answer is: Allow TCP port 443 from 0.0.0.0/0 — Option B is correct because HTTPS uses TCP port 443. Option A is wrong because HTTPS does not use UDP. Option C is wrong because the ALB should not allow all inbound traffic. Option D is wrong because ephemeral ports are for outbound, not inbound.
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.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.
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