The correct answer is that the rule uses negation on the source address. When negation is applied to a source IP in a Palo Alto Networks security policy, the firewall interprets the source as “any IP except that address,” so traffic from 10.1.1.5 is explicitly excluded from matching rule1 and instead falls through to the default deny rule. This is a classic example of how negation in security policies can cause unintended blocks, because the excluded traffic is treated as non-matching and is not implicitly allowed. On the PCNSA exam, this concept tests your understanding of policy evaluation logic and the behavior of negative conditions, often appearing as a trap where a rule appears to cover a host but actually excludes it. A reliable memory tip: “Negation means exclusion, not permission—if you say ‘not 10.1.1.5,’ you are blocking 10.1.1.5 from matching.”
PCNSA Core Concepts Practice Question
This PCNSA practice question tests your understanding of core concepts. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
```
> show security-rule rule1
rule "rule1" {
from untrust;
to trust;
source [10.1.1.0/24];
negate-source yes;
destination any;
application any;
service any;
action allow;
}
```
Refer to the exhibit. A user at IP 10.1.1.5 on the untrust zone is trying to access a server on the trust zone. The traffic is being blocked by a default deny rule instead of being allowed by rule1. What is the most likely reason?
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.
Refer to the exhibit.
```
> show security-rule rule1
rule "rule1" {
from untrust;
to trust;
source [10.1.1.0/24];
negate-source yes;
destination any;
application any;
service any;
action allow;
}
```
A
The rule's destination is set to 'any' which is too broad.
Why wrong: A destination of 'any' does not cause a no-match; it matches all destinations.
B
The rule does not specify any application or service.
Why wrong: The service is set to 'any', so it matches all services. Application 'any' also matches.
C
The rule uses negation on the source address.
The 'negate-source yes' directive means the rule applies to all sources except the specified range 10.1.1.0/24. Since 10.1.1.5 falls in that range, the rule does not match, and traffic hits the default deny rule.
D
The rule has not been committed.
Why wrong: The rule appears in the running configuration via 'show security-rule', indicating it is committed.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The rule uses negation on the source address.
Option C is correct because when a rule uses negation on the source address (e.g., 'not 10.1.1.5'), the firewall treats the source as 'any IP except 10.1.1.5'. This means traffic from 10.1.1.5 will not match the rule, causing it to fall through to the default deny rule. Negation in Palo Alto Networks policies explicitly excludes the specified address from matching, which is a common misconfiguration.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 rule's destination is set to 'any' which is too broad.
Why it's wrong here
A destination of 'any' does not cause a no-match; it matches all destinations.
✗
The rule does not specify any application or service.
Why it's wrong here
The service is set to 'any', so it matches all services. Application 'any' also matches.
✓
The rule uses negation on the source address.
Why this is correct
The 'negate-source yes' directive means the rule applies to all sources except the specified range 10.1.1.0/24. Since 10.1.1.5 falls in that range, the rule does not match, and traffic hits the default deny rule.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The rule has not been committed.
Why it's wrong here
The rule appears in the running configuration via 'show security-rule', indicating it is committed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse negation with a simple 'not' operator, thinking it will allow all traffic except the specified IP, but they overlook that the negated address itself is excluded from matching, causing traffic from that IP to hit the default deny rule.
Trap categories for this question
Command / output trap
The rule appears in the running configuration via 'show security-rule', indicating it is committed.
Detailed technical explanation
How to think about this question
In Palo Alto Networks firewalls, negation on source or destination addresses is implemented by converting the rule into a 'not-equal' match in the packet flow. When a rule uses negation, the firewall must evaluate the source IP against the negated object; if the source matches the negated address, the rule is skipped. This is different from simply omitting the address, as negation actively excludes specific IPs, often leading to unintended blocking when administrators assume negation works like an 'except' clause in a broader allow rule.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Core Concepts — This question tests Core Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The rule uses negation on the source address. — Option C is correct because when a rule uses negation on the source address (e.g., 'not 10.1.1.5'), the firewall treats the source as 'any IP except 10.1.1.5'. This means traffic from 10.1.1.5 will not match the rule, causing it to fall through to the default deny rule. Negation in Palo Alto Networks policies explicitly excludes the specified address from matching, which is a common misconfiguration.
What should I do if I get this PCNSA question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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