Question 1,249 of 2,015

ACLs and CoPP →hardMultiple SelectObjective-mapped

Quick Answer

The answer is that an implicit deny any statement is automatically added at the end of every IPv4 ACL on Cisco IOS. This is correct because ACLs operate on a first-match basis, and if no permit statement matches a packet, the implicit deny ensures the packet is dropped by default, providing a fundamental security boundary. Standard ACLs filter only on the source IP address, while extended ACLs can filter on source and destination IP, protocol, and port numbers, making extended ACLs more granular and preferable for placement closer to the source. On the ENCOR 350-401 exam, this concept tests your understanding of ACL processing order and the critical difference between standard and extended ACLs; a common trap is forgetting that the implicit deny is invisible but always present, or confusing the order of statements. A reliable memory tip is "first match wins, last deny hides"—the implicit deny is the silent guard at the end of every list.

CCNP ACLs and CoPP Practice Question

This 350-401 practice question tests your understanding of acls and copp. 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.

Which three statements about IPv4 ACLs on Cisco IOS are true? (Choose three.)

Question 1hardmulti select

Study the full ACL explanation →

A
Standard ACLs can filter traffic based on source IP address only.
Correct because standard ACLs (numbered 1-99, 1300-1999) examine only the source IP address.
B
Extended ACLs can filter based on source and destination IP addresses, protocol, and port numbers.
Correct because extended ACLs (numbered 100-199, 2000-2699) support these additional match criteria.
C
An implicit deny any statement is automatically added at the end of every ACL.
Correct because all ACLs have an implicit deny any at the end, which drops any traffic not explicitly permitted.
D
ACL entries are processed from bottom to top, with the last match determining the action.
Why wrong: Incorrect because ACL entries are processed sequentially from top to bottom; the first match determines the action.
E
An ACL applied to an inbound interface filters traffic leaving that interface.
Why wrong: Incorrect because an inbound ACL filters traffic entering the interface; an outbound ACL filters traffic leaving the interface.

Full breakdown with real-world context →

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

✓

Standard ACLs can filter traffic based on source IP address only.

Standard ACLs filter only source IP, extended ACLs filter more fields, and the implicit deny is always present. The incorrect options confuse the order of processing or the placement of ACLs.

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.

✓
Standard ACLs can filter traffic based on source IP address only.
Why this is correct
Correct because standard ACLs (numbered 1-99, 1300-1999) examine only the source IP address.
Related concept
Static NAT maps one inside address to one outside address.
✓
Extended ACLs can filter based on source and destination IP addresses, protocol, and port numbers.
Why this is correct
Correct because extended ACLs (numbered 100-199, 2000-2699) support these additional match criteria.
Related concept
Static NAT maps one inside address to one outside address.
✓
An implicit deny any statement is automatically added at the end of every ACL.
Why this is correct
Correct because all ACLs have an implicit deny any at the end, which drops any traffic not explicitly permitted.
Related concept
Static NAT maps one inside address to one outside address.
✗
ACL entries are processed from bottom to top, with the last match determining the action.
Why it's wrong here
Incorrect because ACL entries are processed sequentially from top to bottom; the first match determines the action.
✗
An ACL applied to an inbound interface filters traffic leaving that interface.
Why it's wrong here
Incorrect because an inbound ACL filters traffic entering the interface; an outbound ACL filters traffic leaving the interface.

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.

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 350-401 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 350-401 question test?

ACLs and CoPP — This question tests ACLs and CoPP — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: Standard ACLs can filter traffic based on source IP address only. — Standard ACLs filter only source IP, extended ACLs filter more fields, and the implicit deny is always present. The incorrect options confuse the order of processing or the placement of ACLs.

What should I do if I get this 350-401 question wrong?

What is the key concept behind this question?

Static NAT maps one inside address to one outside address.

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