This CISSP practice question tests your understanding of cissp exam topics. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
! Configuration snippet from router R1
interface GigabitEthernet0/0
ip address 192.168.1.1 255.255.255.0
ip access-group FILTER_ACL in
!
interface GigabitEthernet0/1
ip address 10.0.0.1 255.255.255.0
!
ip access-list extended FILTER_ACL
deny ip any host 10.0.0.100
permit ip any any
!
Refer to the exhibit. A network administrator notices that traffic from the 192.168.1.0/24 network cannot reach the server at 10.0.0.100. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Exhibit
! Configuration snippet from router R1
interface GigabitEthernet0/0
ip address 192.168.1.1 255.255.255.0
ip access-group FILTER_ACL in
!
interface GigabitEthernet0/1
ip address 10.0.0.1 255.255.255.0
!
ip access-list extended FILTER_ACL
deny ip any host 10.0.0.100
permit ip any any
!
A
The server at 10.0.0.100 does not have a default gateway configured.
Why wrong: The exhibit does not provide server configuration; the issue is clearly due to the ACL.
B
The interface GigabitEthernet0/0 has an incorrect IP address.
Why wrong: The IP address and subnet mask appear correct for the 192.168.1.0/24 network.
C
The ACL is applied in the wrong direction on GigabitEthernet0/1.
Why wrong: The ACL is correctly applied inbound on the interface facing the source network. The issue is the explicit deny statement.
D
The ACL contains a deny statement that blocks all traffic to 10.0.0.100.
The ACL has 'deny ip any host 10.0.0.100', which blocks traffic from any source to that specific host.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL contains a deny statement that blocks all traffic to 10.0.0.100.
The ACL FILTER_ACL is applied inbound on interface GigabitEthernet0/0 (facing 192.168.1.0/24). The ACL explicitly denies IP traffic from any source to host 10.0.0.100. Therefore, traffic originating from 192.168.1.0/24 destined to 10.0.0.100 is blocked.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The server at 10.0.0.100 does not have a default gateway configured.
Why it's wrong here
The exhibit does not provide server configuration; the issue is clearly due to the ACL.
✗
The interface GigabitEthernet0/0 has an incorrect IP address.
Why it's wrong here
The IP address and subnet mask appear correct for the 192.168.1.0/24 network.
✗
The ACL is applied in the wrong direction on GigabitEthernet0/1.
Why it's wrong here
The ACL is correctly applied inbound on the interface facing the source network. The issue is the explicit deny statement.
✓
The ACL contains a deny statement that blocks all traffic to 10.0.0.100.
Why this is correct
The ACL has 'deny ip any host 10.0.0.100', which blocks traffic from any source to that specific host.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A SOC analyst notices unusual lateral movement in the network at 2 AM. The IR playbook dictates: identify and contain (isolate the affected machine), then eradicate (remove the malware), then recover (restore from backup), then document. Skipping containment before eradication risks the attacker regaining access. Questions like this test the sequence and rationale of incident response phases.
Visual reference
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CISSP ACL questions on filtering logic and placement.
The correct answer is: The ACL contains a deny statement that blocks all traffic to 10.0.0.100. — The ACL FILTER_ACL is applied inbound on interface GigabitEthernet0/0 (facing 192.168.1.0/24). The ACL explicitly denies IP traffic from any source to host 10.0.0.100. Therefore, traffic originating from 192.168.1.0/24 destined to 10.0.0.100 is blocked.
What should I do if I get this CISSP question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CISSP ACL questions on filtering logic and placement.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Standard ACLs match source addresses.
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