The correct answer is to remove the inbound rule that allows SSH from 0.0.0.0/0 from the security group. This works because AWS security groups are stateful and operate on an allow-list principle—they cannot explicitly deny traffic, so you must simply delete the overly permissive SSH rule to block internet access while keeping the HTTP rule intact. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding that security groups only support allow rules, and a common trap is thinking you can add a deny rule or modify a network ACL, which would affect the entire subnet and potentially block HTTP. Remember, to block SSH from the internet while keeping HTTP, you never add a deny—you just remove the unwanted allow. Memory tip: “Security groups say yes or nothing—they never say no.”
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.
Network Topology
Refer to the exhibit. A security engineer runs the describe-instances command for an EC2 instance. The instance has a public IP address. The security group "allow-ssh-http" has inbound rules that allow SSH from 0.0.0.0/0 and HTTP from 0.0.0.0/0. The engineer wants to block SSH access from the internet while keeping HTTP access. Which change should be made?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove the inbound rule that allows SSH from 0.0.0.0/0 from the security group.
To block SSH from the internet, you should remove the inbound rule that allows SSH from 0.0.0.0/0. You could also modify the source IP range to a specific range, but the simplest is to remove the rule. Adding a deny rule is not possible in security groups; they are allow-only. Changing the network ACL would affect the entire subnet. Removing the public IP would also block HTTP, which is not desired.
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.
✓
Remove the inbound rule that allows SSH from 0.0.0.0/0 from the security group.
Why this is correct
This will block SSH from the internet, as security groups are allow-only.
Add a network ACL rule to deny SSH inbound from 0.0.0.0/0.
Why it's wrong here
Network ACLs apply to the subnet, not just this instance, and would also affect other instances.
✗
Disassociate the public IP address from the instance.
Why it's wrong here
This would also block HTTP access from the internet.
✗
Modify the security group to add a deny rule for SSH from 0.0.0.0/0.
Why it's wrong here
Security groups do not support deny rules.
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 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: Remove the inbound rule that allows SSH from 0.0.0.0/0 from the security group. — To block SSH from the internet, you should remove the inbound rule that allows SSH from 0.0.0.0/0. You could also modify the source IP range to a specific range, but the simplest is to remove the rule. Adding a deny rule is not possible in security groups; they are allow-only. Changing the network ACL would affect the entire subnet. Removing the public IP would also block HTTP, which is not desired.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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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.
Question Discussion
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