The correct answer is that the ACL allows HTTP requests to the web server and permits responses, but blocks the server from initiating new connections to the inside. This occurs because the ACL is applied inbound on the DMZ interface, meaning it filters traffic entering the DMZ from the outside, not traffic leaving it. The permit statement specifically allows TCP port 80 from any source to the web server, so only incoming HTTP requests are permitted; the web server’s return traffic is part of the same established session and is allowed by stateful inspection or outbound rules, but any new outbound connection from the server would be blocked since it is not entering the DMZ interface. On the CISSP exam, this tests your understanding of ACL directionality and the principle of least privilege for DMZ hosts—a common trap is confusing inbound versus outbound filtering or assuming the ACL controls all traffic to and from the server. Remember the memory tip: “Inbound on DMZ controls what comes in, not what goes out; the server can reply but can’t dial out.”
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and 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
description DMZ to Web Server
ip address 10.0.0.1 255.255.255.0
!
interface GigabitEthernet0/2
description Inside to Corp Network
ip address 192.168.1.1 255.255.255.0
!
access-list 100 permit tcp any host 10.0.0.10 eq 80
access-list 100 permit tcp host 10.0.0.10 any established
access-list 100 deny ip any any
!
interface GigabitEthernet0/1
ip access-group 100 in
Refer to the exhibit. The ACL is applied inbound on the DMZ interface. What is the effect of this configuration?
Refer to the exhibit.
interface GigabitEthernet0/1
description DMZ to Web Server
ip address 10.0.0.1 255.255.255.0
!
interface GigabitEthernet0/2
description Inside to Corp Network
ip address 192.168.1.1 255.255.255.0
!
access-list 100 permit tcp any host 10.0.0.10 eq 80
access-list 100 permit tcp host 10.0.0.10 any established
access-list 100 deny ip any any
!
interface GigabitEthernet0/1
ip access-group 100 in
A
The ACL permits all HTTP traffic to the web server and blocks all other traffic.
Why wrong: It permits HTTP but also blocks outbound non-established traffic.
B
The ACL allows HTTP requests to the web server and allows the web server to respond, but blocks it from initiating new connections to the inside.
Correct as explained.
C
The ACL permits the web server to respond to HTTP requests but blocks all other outbound traffic.
Why wrong: It permits established connections, so responses are allowed.
D
The ACL allows the web server to initiate connections to the internal network.
Why wrong: It does not have a permit for new outbound connections from the web server to inside.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL allows HTTP requests to the web server and allows the web server to respond, but blocks it from initiating new connections to the inside.
The ACL is applied inbound on the DMZ interface, meaning it filters traffic entering the DMZ from the outside. The specific permit statement allows HTTP (TCP port 80) traffic from any source to the web server's IP address. Because the ACL is inbound, it only controls traffic arriving at the DMZ interface; return traffic from the web server to the inside is not subject to this ACL (it is evaluated by the outbound ACL on the inside interface or by stateful inspection). Thus, the web server can respond to HTTP requests (which are part of the same session), but it cannot initiate new connections to the inside because those would be outbound from the DMZ and not permitted by the inbound ACL on the DMZ 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.
✗
The ACL permits all HTTP traffic to the web server and blocks all other traffic.
Why it's wrong here
It permits HTTP but also blocks outbound non-established traffic.
✓
The ACL allows HTTP requests to the web server and allows the web server to respond, but blocks it from initiating new connections to the inside.
Why this is correct
Correct as explained.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ACL permits the web server to respond to HTTP requests but blocks all other outbound traffic.
Why it's wrong here
It permits established connections, so responses are allowed.
✗
The ACL allows the web server to initiate connections to the internal network.
Why it's wrong here
It does not have a permit for new outbound connections from the web server to inside.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between inbound and outbound ACL application, tricking candidates into thinking an inbound ACL on the DMZ interface controls outbound traffic from the DMZ, when in fact it only controls traffic entering the DMZ.
Detailed technical explanation
How to think about this question
In Cisco IOS, ACLs applied inbound on an interface filter traffic before it is processed by the router, while outbound ACLs filter traffic after routing. For stateful firewalls (like ASA), the concept of 'established' connections is handled by the state table, but in a pure ACL scenario (e.g., extended ACL on a router), return traffic for a permitted session is allowed only if the ACL includes a permit statement for the return direction (e.g., permit tcp host web-server any established) or if the router is configured with reflexive ACLs. In this question, the ACL is a standard or extended ACL without stateful inspection, so the web server's responses are allowed only because they are part of the same TCP session (the router tracks TCP state via the 'established' keyword if present, or by the fact that the response is a reply to a permitted request).
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 CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL allows HTTP requests to the web server and allows the web server to respond, but blocks it from initiating new connections to the inside. — The ACL is applied inbound on the DMZ interface, meaning it filters traffic entering the DMZ from the outside. The specific permit statement allows HTTP (TCP port 80) traffic from any source to the web server's IP address. Because the ACL is inbound, it only controls traffic arriving at the DMZ interface; return traffic from the web server to the inside is not subject to this ACL (it is evaluated by the outbound ACL on the inside interface or by stateful inspection). Thus, the web server can respond to HTTP requests (which are part of the same session), but it cannot initiate new connections to the inside because those would be outbound from the DMZ and not permitted by the inbound ACL on the DMZ interface.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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