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Question 16 of 529

Policy Evaluation and Management →mediumMultiple ChoiceObjective-mapped

Allow Internal, Deny External

This PCNSA practice question tests your understanding of policy evaluation and management. 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.

A company needs to restrict access to a critical server from external IP addresses, but internal users should have full access. Which rule structure should be used?

A
Create a deny rule for external IP addresses, then an allow rule for internal.
Why wrong: If the deny rule matches external IPs first, it is fine; but if the external IP range is broad, it might also match internal if not careful. More importantly, the allow rule for internal must be before the deny to ensure internal traffic is not denied.
B
Place the allow rule after the deny rule.
Why wrong: If the deny rule is first, it would block all traffic, including internal, before reaching the allow rule.
C
Create an allow rule for internal source addresses, then a deny rule for any source.
Internal traffic is allowed by the first rule, and all other (external) traffic is denied by the second rule.
D
Create a single rule with a 'Deny' action and apply a user-ID condition.
Why wrong: User-ID is for user-based policies, and a single deny rule would block all, not just external.

Full breakdown with real-world context →

Quick Answer

The answer is to create an allow rule for internal source addresses, then a deny rule for any source. This structure works because Palo Alto Networks firewalls evaluate security rules from the top down, so the specific allow for internal traffic is matched first, and the subsequent catch-all deny blocks everything else, including external IPs. On the PCNSA exam, this tests your understanding of rule ordering and source-based access control, a common trap being that a broad deny rule placed before the allow will block all traffic, including internal users. A reliable memory tip is "specific allow first, then deny the rest," ensuring internal external access control is enforced correctly without disrupting legitimate 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

✓

Create an allow rule for internal source addresses, then a deny rule for any source.

Correct answer is C. Rule order is critical: Palo Alto firewalls evaluate rules top-down, and the first match is applied. By placing an allow rule for internal source addresses first, internal traffic is permitted. Then a subsequent deny rule for any source blocks all other traffic, including external IPs. Option A is incorrect because creating a deny rule for external IPs first would block external traffic but does not guarantee internal access if the deny rule is overly broad or if any later rule could inadvertently allow external traffic. Option B is incorrect because placing the allow rule after the deny rule would cause the deny rule to match all traffic first, including internal, since the deny rule typically covers any source. Option D is incorrect because a single deny rule with a user-ID condition cannot differentiate between internal and external users unless all internal users are authenticated, which is impractical for basic IP-based access control.

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.

✗
Create a deny rule for external IP addresses, then an allow rule for internal.
Why it's wrong here
If the deny rule matches external IPs first, it is fine; but if the external IP range is broad, it might also match internal if not careful. More importantly, the allow rule for internal must be before the deny to ensure internal traffic is not denied.
✗
Place the allow rule after the deny rule.
Why it's wrong here
If the deny rule is first, it would block all traffic, including internal, before reaching the allow rule.
✓
Create an allow rule for internal source addresses, then a deny rule for any source.
Why this is correct
Internal traffic is allowed by the first rule, and all other (external) traffic is denied by the second rule.
Related concept
Standard ACLs match source addresses.
✗
Create a single rule with a 'Deny' action and apply a user-ID condition.
Why it's wrong here
User-ID is for user-based policies, and a single deny rule would block all, not just external.

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

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 PCNSA ACL questions on filtering logic and placement.

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FAQ

Questions learners often ask

What does this PCNSA question test?

Policy Evaluation and Management — This question tests Policy Evaluation and Management — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: Create an allow rule for internal source addresses, then a deny rule for any source. — Correct answer is C. Rule order is critical: Palo Alto firewalls evaluate rules top-down, and the first match is applied. By placing an allow rule for internal source addresses first, internal traffic is permitted. Then a subsequent deny rule for any source blocks all other traffic, including external IPs. Option A is incorrect because creating a deny rule for external IPs first would block external traffic but does not guarantee internal access if the deny rule is overly broad or if any later rule could inadvertently allow external traffic. Option B is incorrect because placing the allow rule after the deny rule would cause the deny rule to match all traffic first, including internal, since the deny rule typically covers any source. Option D is incorrect because a single deny rule with a user-ID condition cannot differentiate between internal and external users unless all internal users are authenticated, which is impractical for basic IP-based access control.

What should I do if I get this PCNSA question wrong?

What is the key concept behind this question?

Standard ACLs match source addresses.

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