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Question 1,089 of 1,738

Infrastructure Security →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is to modify the existing security group rule, changing the source from 0.0.0.0/0 to 203.0.113.0/24. This is the most efficient approach because security groups are stateful and operate on a per-rule basis; editing the existing rule directly replaces the overly permissive source with the restricted IP range, instantly closing the wide-open SSH access without disrupting other inbound or outbound rules. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding that security group rules are mutable and that adding a new rule does not override an existing one—a common trap where candidates mistakenly think a more specific rule takes precedence. Remember, security groups evaluate all rules before allowing traffic, so you must explicitly remove or modify the overly broad rule. Memory tip: "Edit, don't add—overly permissive rules stay bad."

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 company is using Amazon EC2 instances in a VPC with a security group that allows inbound SSH from 0.0.0.0/0. A security engineer needs to restrict SSH access to only the company's public IP range (203.0.113.0/24) while maintaining all other existing rules. What is the MOST efficient way to accomplish this?

Question 1mediummultiple choice

Full question →

A
Disable SSH and use AWS Systems Manager Session Manager to connect to instances.
Why wrong: Session Manager provides secure access but does not address the existing SSH rule; the question asks to restrict SSH.
B
Create a network ACL with an inbound rule allowing SSH from 203.0.113.0/24 and deny all other traffic.
Why wrong: NACLs are stateless and would affect all traffic; this is more complex and less efficient.
C
Modify the existing security group rule to change the source from 0.0.0.0/0 to 203.0.113.0/24.
Modifying the existing rule directly updates the source to the required CIDR, removing the open access.
D
Create a new security group rule allowing SSH from 203.0.113.0/24 and keep the existing rule.
Why wrong: Keeping the existing 0.0.0.0/0 rule still allows all inbound SSH traffic.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Modify the existing security group rule to change the source from 0.0.0.0/0 to 203.0.113.0/24.

Option B is correct because modifying the existing security group rule to change the source CIDR is the most direct method. Option A is wrong because adding a new rule doesn't remove the open rule. Option C is wrong because NACLs are stateless and would require additional rules. Option D is wrong because System Manager Session Manager does not replace the need for SSH restrictions.

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.

✗
Disable SSH and use AWS Systems Manager Session Manager to connect to instances.
Why it's wrong here
Session Manager provides secure access but does not address the existing SSH rule; the question asks to restrict SSH.
✗
Create a network ACL with an inbound rule allowing SSH from 203.0.113.0/24 and deny all other traffic.
Why it's wrong here
NACLs are stateless and would affect all traffic; this is more complex and less efficient.
✓
Modify the existing security group rule to change the source from 0.0.0.0/0 to 203.0.113.0/24.
Why this is correct
Modifying the existing rule directly updates the source to the required CIDR, removing the open access.
Related concept
CIDR notation defines the prefix length.
✗
Create a new security group rule allowing SSH from 203.0.113.0/24 and keep the existing rule.
Why it's wrong here
Keeping the existing 0.0.0.0/0 rule still allows all inbound SSH traffic.

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.

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FAQ

Questions learners often ask

What does this SCS-C02 question test?

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: Modify the existing security group rule to change the source from 0.0.0.0/0 to 203.0.113.0/24. — Option B is correct because modifying the existing security group rule to change the source CIDR is the most direct method. Option A is wrong because adding a new rule doesn't remove the open rule. Option C is wrong because NACLs are stateless and would require additional rules. Option D is wrong because System Manager Session Manager does not replace the need for SSH restrictions.

What should I do if I get this SCS-C02 question wrong?

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.