Quick Answer
The answer is to modify the network ACL to deny traffic from that IP, as this is the only AWS-native option among the choices that can explicitly block a malicious IP at the subnet level. A network ACL is a stateless firewall that evaluates traffic entering or leaving a subnet, and because it supports explicit deny rules, it can effectively drop packets from a known bad source before they reach the EC2 instance. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this question tests your understanding of the critical difference between security groups, which are stateful and only support allow rules, and network ACLs, which are stateless and support both allow and deny rules. A common trap is assuming a security group can block an IP, but since it cannot deny traffic, the correct tool for subnet-level IP blocking is the network ACL. Memory tip: think "NACL denies, SG allows only" — if you need to block a specific IP, go with the ACL.
This SOA-C02 practice question tests your understanding of security and compliance. 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 SysOps administrator notices that an EC2 instance running a web server is receiving unexpected traffic from an IP address that is known to be malicious. The administrator wants to block this IP address at the instance level. Which solution should be used?
Modify the network ACL to deny traffic from that IP.
NACLs support deny rules and can block IPs at subnet level.
Use AWS WAF to block the IP address.
Why wrong: WAF is for web application firewall, not instance-level.
Install a third-party firewall on the instance.
Why wrong: Not an AWS-native solution.
Update the security group to deny traffic from that IP.
Why wrong: Security groups only support allow rules.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Modify the network ACL to deny traffic from that IP.
Option D is correct because a network ACL is stateless and can block inbound traffic at the subnet level, but for instance-level blocking, a security group or host-based firewall is needed. However, among the options, a network ACL can block the IP at the subnet level, which is the closest AWS-native solution without installing software. Option A is wrong because security groups do not support deny rules. Option B is wrong because NACLs can block IPs. Option C is wrong because WAF is for web application layer, not IP blocking at instance level.
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.
Modify the network ACL to deny traffic from that IP.
Use AWS WAF to block the IP address.
Why it's wrong here
WAF is for web application firewall, not instance-level.
Install a third-party firewall on the instance.
Why it's wrong here
Not an AWS-native solution.
Update the security group to deny traffic from that IP.
Why it's wrong here
Security groups only support allow rules.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Security and Compliance — study guide chapter
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FAQ
Security and Compliance — This question tests Security and Compliance — CIDR notation defines the prefix length..
The correct answer is: Modify the network ACL to deny traffic from that IP. — Option D is correct because a network ACL is stateless and can block inbound traffic at the subnet level, but for instance-level blocking, a security group or host-based firewall is needed. However, among the options, a network ACL can block the IP at the subnet level, which is the closest AWS-native solution without installing software. Option A is wrong because security groups do not support deny rules. Option B is wrong because NACLs can block IPs. Option C is wrong because WAF is for web application layer, not IP blocking at instance level.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-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 SOA-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 SOA-C02 exam.
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