Question 752 of 1,389
CCNA Network Services and Security Practice Question
A network engineer notices that hosts in the 192.168.2.0/24 network connected to router R1's GigabitEthernet0/1 interface cannot reach the Internet. R1 has a standard ACL 10 configured as 'access-list 10 permit 192.168.1.0 0.0.0.255' and applied inbound on interface GigabitEthernet0/0, which connects to the 192.168.1.0/24 LAN. What is the most likely cause?
⚠ Common exam trap
Many exam-takers assume ACLs filter outbound traffic from the local network, but Cisco tests the nuance that an inbound ACL on the WAN-facing interface filters return traffic, not the original outbound traffic.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL is filtering return traffic from the Internet that enters G0/0, because it is applied inbound on that interface instead of outbound.
ACL 10 is applied inbound on GigabitEthernet0/0, which connects to the 192.168.1.0/24 LAN. When traffic from the Internet returns to hosts in the 192.168.2.0/24 network, it enters G0/0 inbound and is evaluated against ACL 10, which only permits source addresses from 192.168.1.0/24. The implicit deny at the end of the ACL then blocks all return traffic destined for 192.168.2.0/24, preventing those hosts from reaching the Internet.
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 implicit deny at the end of ACL 10 is blocking all outbound traffic from the 192.168.2.0/24 network.
Why it's wrong here
Standard ACLs evaluate packets only as they enter the interface on which the ACL is applied. Because ACL 10 is applied inbound on G0/0, it inspects packets arriving from the Internet, not packets that the router is forwarding out G0/0 toward the Internet. Outbound traffic sourced from 192.168.2.0/24 would egress G0/0 (or another interface) and would never match this inbound ACL, so the implicit deny at the end of ACL 10 cannot block that traffic. The deny only rejects inbound packets that do not match the permit statement, making it irrelevant to outbound flow.
- ✓
The ACL is filtering return traffic from the Internet that enters G0/0, because it is applied inbound on that interface instead of outbound.
Why this is correct
Inbound ACLs on G0/0 inspect packets arriving from the Internet. The ACL permits only source 192.168.1.0/24, so return packets from Internet hosts with random source IPs are denied by the implicit deny, breaking connectivity for 192.168.2.0/24 hosts.
- ✗
The router is not performing inter-VLAN routing between the 192.168.1.0 and 192.168.2.0 networks.
Why it's wrong here
Inter-VLAN routing relies on the routing table, not ACLs, to forward packets between directly connected subnets. The ACL only filters traffic passing through the interface where it is applied and does not disable routing between the two LANs.
- ✗
The ACL is missing a permit statement for the 192.168.2.0/24 network to allow traffic from that subnet.
Why it's wrong here
Even if a permit for 192.168.2.0/24 were added, the ACL is applied inbound on G0/0. Traffic from 192.168.2.0/24 going to the Internet would exit out G0/0, not enter, so it would not be evaluated by this ACL. The missing permit is not the root cause; the direction is wrong.
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.
✓The ACL is filtering return traffic from the Internet that enters G0/0, because it is applied inbound on that interface instead of outbound.Correct answer▾
Why this is correct
Inbound ACLs on G0/0 inspect packets arriving from the Internet. The ACL permits only source 192.168.1.0/24, so return packets from Internet hosts with random source IPs are denied by the implicit deny, breaking connectivity for 192.168.2.0/24 hosts.
✗The implicit deny at the end of ACL 10 is blocking all outbound traffic from the 192.168.2.0/24 network.Wrong answer — click to see why▾
Why this is wrong here
Misunderstanding of ACL direction leads candidates to think that the implicit deny blocks any traffic leaving the interface.
✗The router is not performing inter-VLAN routing between the 192.168.1.0 and 192.168.2.0 networks.Wrong answer — click to see why▾
Why this is wrong here
Confusing ACL filtering with routing functionality; ACLs do not prevent the router from routing between connected subnets unless they explicitly deny the traffic on the appropriate interface and direction.
✗The ACL is missing a permit statement for the 192.168.2.0/24 network to allow traffic from that subnet.Wrong answer — click to see why▾
Why this is wrong here
Candidates often try to add a permit for the source subnet of the initiating traffic, neglecting the direction of the ACL. Because the ACL is inbound on the egress interface, outbound traffic is not filtered.
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?”
Visual reference
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Last reviewed: Jul 4, 2026
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