- A
The original standard ACL only had a permit statement, so after removal the permit still takes effect because the ACL remains in the running configuration.
Why wrong: Removing the access-group command from the interface stops all filtering. An ACL defined without being applied does not affect traffic.
- B
The ip access-group command on the interface remains but is missing the referenced ACL, causing the router to default to denying all ingress traffic except the previously permitted 10.0.0.0/8.
Why wrong: When an ACL is removed from an interface, the access-group configuration is also removed entirely. There is no 'dangling' reference that would selectively permit RFC 1918 addresses.
- C
Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.
Before removal, the applied ACL permitted only 10.0.0.0/8 and denied everything else (implicit deny all), which correctly blocked spoofed RFC 1918 traffic. Once the ACL is de-applied, the interface has no access list, so all traffic is permitted, including the previously blocked spoofed packets.
- D
The ACL was reapplied in the outbound direction instead of inbound, so it now blocks traffic leaving the interface but not entering it.
Why wrong: The scenario states the ACL was removed, not reapplied in a different direction. No outbound filtering is mentioned, and a standard ACL applied outbound would not block ingress traffic.
Quick Answer
The answer is that removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic. This happens because when you apply the 'no ip access-group' command, the interface reverts to its default state of permitting all traffic, meaning the previous ACL’s implicit deny all rule no longer filters packets. On the CCNA 200-301 v2 exam, this concept tests your understanding that ACLs do not persist on an interface after removal—a common trap is assuming the implicit deny remains active even without the ACL applied. The scenario often appears in troubleshooting questions where spoofed RFC 1918 addresses suddenly pass through, highlighting that ACL removal is not a configuration change but a complete removal of filtering logic. Memory tip: "No ACL, no deny"—if the access-group is gone, the implicit deny is gone too, so the router defaults to permit all.
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A network engineer notices that after removing a standard ACL that was applied inbound on the internet-facing interface, the router is now receiving IP packets from the internet with source IP addresses in the 10.0.0.0/8 range, which were previously blocked. 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.
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
Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.
When an ACL is removed from an interface using 'no ip access-group', the interface reverts to its default behavior of permitting all traffic. The previous ACL's implicit deny all no longer applies, so spoofed RFC 1918 source addresses that were once blocked are now allowed to enter.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 original standard ACL only had a permit statement, so after removal the permit still takes effect because the ACL remains in the running configuration.
Why it's wrong here
Removing the access-group command from the interface stops all filtering. An ACL defined without being applied does not affect traffic.
- ✗
The ip access-group command on the interface remains but is missing the referenced ACL, causing the router to default to denying all ingress traffic except the previously permitted 10.0.0.0/8.
Why it's wrong here
When an ACL is removed from an interface, the access-group configuration is also removed entirely. There is no 'dangling' reference that would selectively permit RFC 1918 addresses.
- ✓
Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.
Why this is correct
Before removal, the applied ACL permitted only 10.0.0.0/8 and denied everything else (implicit deny all), which correctly blocked spoofed RFC 1918 traffic. Once the ACL is de-applied, the interface has no access list, so all traffic is permitted, including the previously blocked spoofed packets.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The ACL was reapplied in the outbound direction instead of inbound, so it now blocks traffic leaving the interface but not entering it.
Why it's wrong here
The scenario states the ACL was removed, not reapplied in a different direction. No outbound filtering is mentioned, and a standard ACL applied outbound would not block ingress traffic.
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.
✓Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.Correct answer▾
Why this is correct
Before removal, the applied ACL permitted only 10.0.0.0/8 and denied everything else (implicit deny all), which correctly blocked spoofed RFC 1918 traffic. Once the ACL is de-applied, the interface has no access list, so all traffic is permitted, including the previously blocked spoofed packets.
✗The original standard ACL only had a permit statement, so after removal the permit still takes effect because the ACL remains in the running configuration.Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that the ACL itself, if still configured, continues to filter traffic even when not applied to an interface.
✗The ip access-group command on the interface remains but is missing the referenced ACL, causing the router to default to denying all ingress traffic except the previously permitted 10.0.0.0/8.Wrong answer — click to see why▾
Why this is wrong here
A common misunderstanding is that the access-group line can persist without a valid ACL and cause some default behavior; in fact the entire command is removed.
✗The ACL was reapplied in the outbound direction instead of inbound, so it now blocks traffic leaving the interface but not entering it.Wrong answer — click to see why▾
Why this is wrong here
Some candidates may confuse direction changes with removal and assume the ACL is still filtering traffic in some way, but the symptom clearly indicates no filtering at all.
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
Removing the access-group command from the interface stops all filtering. An ACL defined without being applied does not affect traffic.
Scenario analysis trap
The scenario states the ACL was removed, not reapplied in a different direction. No outbound filtering is mentioned, and a standard ACL applied outbound would not block ingress traffic.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Services and Security — This question tests Network Services and Security — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic. — When an ACL is removed from an interface using 'no ip access-group', the interface reverts to its default behavior of permitting all traffic. The previous ACL's implicit deny all no longer applies, so spoofed RFC 1918 source addresses that were once blocked are now allowed to enter.
What should I do if I get this 200-301 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 14, 2026
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