The answer is that ACL 100 is applied outbound on the internal interface, so it fails to filter traffic from internal users. The core issue is a misunderstanding of ACL direction troubleshooting: outbound vs inbound. An outbound ACL on the internal interface only inspects packets leaving that interface, but internal users’ web traffic enters the router through that same interface, meaning it must be filtered inbound on the internal interface or outbound on the external interface to control the traffic flow. On the ISC2 Certified in Cybersecurity CC exam, this tests your grasp of where ACLs are evaluated in the packet path—a common trap is assuming “outbound” on any interface controls traffic leaving the network, when it actually controls traffic leaving that specific interface. A reliable memory tip is “inbound on the source side, outbound on the destination side”—if you want to filter traffic as it enters the router from internal users, apply the ACL inbound on their interface.
ISC2 CC Network Security Practice Question
This CC practice question tests your understanding of network security. 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
Refer to the exhibit.
interface GigabitEthernet0/1
ip address 192.168.1.1 255.255.255.0
ip access-group OUTBOUND out
!
interface GigabitEthernet0/2
ip address 10.0.0.1 255.255.255.0
ip access-group INBOUND in
!
access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 80
access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 443
access-list 100 deny ip any any
!
access-list 110 permit tcp any host 10.0.0.10 eq 22
access-list 110 permit icmp any host 10.0.0.10 echo-reply
access-list 110 deny ip any any
Refer to the exhibit. An administrator configures the above ACLs on a router. The goal is to allow internal users (192.168.1.0/24) to browse the web, and to allow SSH management from the internet to a server at 10.0.0.10. However, users report that they cannot browse external websites. What is the most likely reason?
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.
Refer to the exhibit.
interface GigabitEthernet0/1
ip address 192.168.1.1 255.255.255.0
ip access-group OUTBOUND out
!
interface GigabitEthernet0/2
ip address 10.0.0.1 255.255.255.0
ip access-group INBOUND in
!
access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 80
access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 443
access-list 100 deny ip any any
!
access-list 110 permit tcp any host 10.0.0.10 eq 22
access-list 110 permit icmp any host 10.0.0.10 echo-reply
access-list 110 deny ip any any
A
ACL 100 is applied outbound on the internal interface, so it does not filter outbound traffic from internal users
The access-group is applied outbound on Gi0/1; traffic from internal users to the internet exits Gi0/1 inbound, so ACL 100 is not checked.
B
ACL 100 does not permit DNS traffic, so users cannot resolve domain names
Why wrong: The stem mentions browsing, which could be directly via IP; also DNS is not necessary if using IP addresses.
C
ACL 110 blocks the return traffic from the internet
Why wrong: ACL 110 is applied inbound on Gi0/2; it permits SSH and ICMP echo replies, but does not block return HTTP traffic.
D
The implicit deny at the end of ACL 100 blocks established connections
Why wrong: ACL 100 permits HTTP/HTTPS and then denies all; established connections are not automatically allowed unless stateful inspection is enabled.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ACL 100 is applied outbound on the internal interface, so it does not filter outbound traffic from internal users
ACL 100 is applied outbound on the internal interface (e.g., GigabitEthernet0/0). Outbound ACLs filter traffic leaving the interface, but internal users' web traffic originates from the internal network and must be allowed inbound on the internal interface (or outbound on the external interface). Since ACL 100 is applied outbound on the internal interface, it only filters traffic that has already been routed out of that interface, which is not the path for internal users' outbound web requests. The correct approach is to apply the ACL inbound on the internal interface or outbound on the external interface.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
ACL 100 is applied outbound on the internal interface, so it does not filter outbound traffic from internal users
Why this is correct
The access-group is applied outbound on Gi0/1; traffic from internal users to the internet exits Gi0/1 inbound, so ACL 100 is not checked.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
ACL 100 does not permit DNS traffic, so users cannot resolve domain names
Why it's wrong here
The stem mentions browsing, which could be directly via IP; also DNS is not necessary if using IP addresses.
✗
ACL 110 blocks the return traffic from the internet
Why it's wrong here
ACL 110 is applied inbound on Gi0/2; it permits SSH and ICMP echo replies, but does not block return HTTP traffic.
✗
The implicit deny at the end of ACL 100 blocks established connections
Why it's wrong here
ACL 100 permits HTTP/HTTPS and then denies all; established connections are not automatically allowed unless stateful inspection is enabled.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the concept that ACL direction is relative to the interface, not the traffic source; the trap here is that candidates assume an outbound ACL on the internal interface will filter traffic leaving the internal network, but it actually filters traffic leaving the router through that interface, which is the opposite direction for internal users' outbound traffic.
Detailed technical explanation
How to think about this question
Cisco ACLs are stateless, meaning they evaluate each packet independently without tracking connection state. For outbound web traffic, the return packets (from web servers) are considered new inbound packets and must be permitted by an inbound ACL on the external interface or by an established keyword (which allows TCP packets with ACK or RST bits set). In real-world scenarios, applying ACLs in the wrong direction is a common misconfiguration that leads to asymmetric traffic filtering, often requiring careful review of traffic flow and interface direction.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CC question in full detail.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ACL 100 is applied outbound on the internal interface, so it does not filter outbound traffic from internal users — ACL 100 is applied outbound on the internal interface (e.g., GigabitEthernet0/0). Outbound ACLs filter traffic leaving the interface, but internal users' web traffic originates from the internal network and must be allowed inbound on the internal interface (or outbound on the external interface). Since ACL 100 is applied outbound on the internal interface, it only filters traffic that has already been routed out of that interface, which is not the path for internal users' outbound web requests. The correct approach is to apply the ACL inbound on the internal interface or outbound on the external interface.
What should I do if I get this CC question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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