CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and 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.
Network Topology
You are connected to R1. The network consists of three routers: R1, R2, and R3. R1's G0/0 connects to R2 (10.0.0.0/30), and R1's G0/1 connects to R3 (10.0.1.0/30). A server at 203.0.113.100 on R2's LAN must be reachable from R3's LAN (203.0.113.0/24) via ICMP, but all other traffic from R3 to R2 must be blocked. The current ACL on R1 is too permissive, allowing all traffic. Configure and apply a standard ACL to permit only ICMP echo requests from R3 to the server, with the implicit deny blocking everything else.
R1# show running-config | section interface
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.252
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 10.0.1.1 255.255.255.252
duplex auto
speed auto
!
R1# show running-config | section access-list
access-list 10 permit any
!
interface GigabitEthernet0/0
ip access-group 10 in
!
A
Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' and apply it inbound on G0/1.
This is correct because the requirement is to allow only ICMP echo requests from R3's LAN to the server. Since standard ACLs cannot filter by protocol, an extended ACL is needed. Applying it inbound on G0/1 (facing R3) filters traffic before it enters R1, and the implicit deny blocks all other traffic.
B
Remove ACL 10 from G0/0, delete ACL 10, then configure standard ACL 10 with 'permit 203.0.113.0 0.0.0.255' and apply it inbound on G0/1.
Why wrong: This is incorrect because a standard ACL can only filter based on source IP address, not on protocol type. It would permit all traffic from the 203.0.113.0/24 network, not just ICMP echo requests.
C
Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp host 203.0.113.100 203.0.113.0 0.0.0.255 echo' and apply it inbound on G0/0.
Why wrong: This is incorrect because the source and destination are reversed. The ACL should permit ICMP echo requests from the source network (203.0.113.0/24) to the destination host (203.0.113.100), not the other way around. Also, applying it inbound on G0/0 would filter traffic from R2, not from R3.
D
Keep the existing ACL 10 applied inbound on G0/0, but add an extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' applied inbound on G0/1.
Why wrong: This is incorrect because the existing ACL 10 permits all traffic inbound on G0/0, which would still allow all traffic from R3 to R2 (since traffic from R3 enters R1 via G0/1, but then exits via G0/0; the inbound ACL on G0/0 does not filter traffic exiting R1). The requirement is to block all other traffic, so the permissive ACL must be removed.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' and apply it inbound on G0/1.
The current ACL 10 permits all traffic inbound on G0/0, which is too permissive. The requirement is to allow only ICMP echo requests from R3's LAN (source 203.0.113.0/24) to the server at 203.0.113.100. A standard ACL uses source IP only. First, remove the existing ACL from the interface with 'no ip access-group 10 in' on G0/0. Then delete ACL 10 with 'no access-list 10'. Create a new standard ACL that permits ICMP echo requests; since standard ACLs cannot filter by protocol, we must use an extended ACL. So we configure an extended ACL (e.g., 100) with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' and apply it inbound on G0/1 (the interface facing R3) to filter traffic before it enters R1. The implicit deny will block all other traffic from R3 to R2.
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.
✓
Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' and apply it inbound on G0/1.
Why this is correct
This is correct because the requirement is to allow only ICMP echo requests from R3's LAN to the server. Since standard ACLs cannot filter by protocol, an extended ACL is needed. Applying it inbound on G0/1 (facing R3) filters traffic before it enters R1, and the implicit deny blocks all other traffic.
Related concept
Standard ACLs match source addresses.
✗
Remove ACL 10 from G0/0, delete ACL 10, then configure standard ACL 10 with 'permit 203.0.113.0 0.0.0.255' and apply it inbound on G0/1.
Why it's wrong here
This is incorrect because a standard ACL can only filter based on source IP address, not on protocol type. It would permit all traffic from the 203.0.113.0/24 network, not just ICMP echo requests.
✗
Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp host 203.0.113.100 203.0.113.0 0.0.0.255 echo' and apply it inbound on G0/0.
Why it's wrong here
This is incorrect because the source and destination are reversed. The ACL should permit ICMP echo requests from the source network (203.0.113.0/24) to the destination host (203.0.113.100), not the other way around. Also, applying it inbound on G0/0 would filter traffic from R2, not from R3.
✗
Keep the existing ACL 10 applied inbound on G0/0, but add an extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' applied inbound on G0/1.
Why it's wrong here
This is incorrect because the existing ACL 10 permits all traffic inbound on G0/0, which would still allow all traffic from R3 to R2 (since traffic from R3 enters R1 via G0/1, but then exits via G0/0; the inbound ACL on G0/0 does not filter traffic exiting R1). The requirement is to block all other traffic, so the permissive ACL must be removed.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' and apply it inbound on G0/1.Correct answer▾
Why this is correct
This is correct because the requirement is to allow only ICMP echo requests from R3's LAN to the server. Since standard ACLs cannot filter by protocol, an extended ACL is needed. Applying it inbound on G0/1 (facing R3) filters traffic before it enters R1, and the implicit deny blocks all other traffic.
✗Remove ACL 10 from G0/0, delete ACL 10, then configure standard ACL 10 with 'permit 203.0.113.0 0.0.0.255' and apply it inbound on G0/1.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that standard ACLs cannot filter by protocol (ICMP). They only match source IP addresses.
Why candidates choose this
Candidates might pick this because they think a standard ACL is sufficient since the requirement is to permit traffic from a specific source network, forgetting that the protocol must also be filtered.
✗Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp host 203.0.113.100 203.0.113.0 0.0.0.255 echo' and apply it inbound on G0/0.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the ACL statement has the source and destination swapped, and it is applied on the wrong interface (G0/0 instead of G0/1).
Why candidates choose this
Candidates might confuse the source and destination, thinking the server is the source of the ICMP echo request, or they might apply the ACL on the interface closest to the destination.
✗Keep the existing ACL 10 applied inbound on G0/0, but add an extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' applied inbound on G0/1.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the existing ACL 10 is too permissive and is applied on the wrong interface (G0/0 inbound) to filter traffic from R3 to R2. It must be removed to enforce the new restrictions.
Why candidates choose this
Candidates might think that adding a more specific ACL on G0/1 will override the permissive ACL on G0/0, not realizing that ACLs are processed independently on each interface and direction.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 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 200-301 ACL questions on filtering logic and placement.
Network Services and Security — This question tests Network Services and Security — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Remove ACL 10 from G0/0, delete ACL 10, then configure extended ACL 100 with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' and apply it inbound on G0/1. — The current ACL 10 permits all traffic inbound on G0/0, which is too permissive. The requirement is to allow only ICMP echo requests from R3's LAN (source 203.0.113.0/24) to the server at 203.0.113.100. A standard ACL uses source IP only. First, remove the existing ACL from the interface with 'no ip access-group 10 in' on G0/0. Then delete ACL 10 with 'no access-list 10'. Create a new standard ACL that permits ICMP echo requests; since standard ACLs cannot filter by protocol, we must use an extended ACL. So we configure an extended ACL (e.g., 100) with 'permit icmp 203.0.113.0 0.0.0.255 host 203.0.113.100 echo' and apply it inbound on G0/1 (the interface facing R3) to filter traffic before it enters R1. The implicit deny will block all other traffic from R3 to R2.
What should I do if I get this 200-301 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 200-301 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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