Quick Answer
The answer is 203.0.113.21. In a one-to-one NAT IP calculation, the mapping is determined by the subnet offset: the internal subnet 10.0.1.0/28 spans addresses 10.0.1.0 through 10.0.1.15, and the external pool 203.0.113.16/28 spans 203.0.113.16 through 203.0.113.31. Since host 10.0.1.5 is the 6th usable IP (offset 5 from the network address), it maps directly to the 6th IP in the external pool, which is 203.0.113.16 plus 5, yielding 203.0.113.21. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this concept tests your understanding of static one-to-one IP Pool behavior, where the internal-to-external translation is purely positional rather than based on a dynamic or PAT algorithm. A common trap is forgetting to count from the network address (not the first usable host) or misapplying the offset to the broadcast address. Memory tip: think of it as “same offset, different subnet”—the host’s position within its internal range is identical to its position within the external range.
This NSE4 practice question tests your understanding of firewall policies and nat. 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.
An admin configures a one-to-one IP Pool to map 10.0.1.0/28 to 203.0.113.16/28. A host with IP 10.0.1.5 initiates a connection to the internet. Which source IP will be used for the translated packet?
203.0.113.20
Why wrong: .20 would map to internal .4, not .5.
203.0.113.21
The internal .5 maps to external .21 (16+5).
203.0.113.5
Why wrong: The mapping is based on host offset, not direct replacement.
203.0.113.16
Why wrong: .16 is the first address of the external range, but it maps to internal .0 (network address), not .5.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
203.0.113.21
In a one-to-one IP Pool NAT configuration, the mapping is based on the subnet offset. The internal subnet 10.0.1.0/28 has 16 addresses (10.0.1.0–10.0.1.15), and the external pool 203.0.113.16/28 also has 16 addresses (203.0.113.16–203.0.113.31). The host 10.0.1.5 is the 6th usable IP (offset 5 from the network address), so it maps to the 6th IP in the external pool: 203.0.113.16 + 5 = 203.0.113.21.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
203.0.113.20
Why it's wrong here
.20 would map to internal .4, not .5.
203.0.113.21
Why this is correct
The internal .5 maps to external .21 (16+5).
Related concept
Read the scenario before looking for a memorised answer.
203.0.113.5
Why it's wrong here
The mapping is based on host offset, not direct replacement.
203.0.113.16
Why it's wrong here
.16 is the first address of the external range, but it maps to internal .0 (network address), not .5.
Common exam traps
The trap here is that candidates often mistakenly add the host portion of the internal IP (e.g., .5) directly to the external network address (e.g., 203.0.113.16 + .5 = 203.0.113.21) but then incorrectly select 203.0.113.20 due to off-by-one errors, or they confuse the mapping with dynamic PAT where the source port is translated instead of the IP.
Detailed technical explanation
One-to-one IP Pool NAT (also called static NAT with a pool) performs a fixed, deterministic translation where each internal IP is permanently mapped to a specific external IP based on the offset within the subnet. This is commonly used when an organization has a block of public IPs and needs to provide each internal host with a consistent public identity for logging or policy enforcement. The mapping does not use PAT (port translation), so each internal host consumes an entire public IP, which can be inefficient for large networks.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
The correct answer is: 203.0.113.21 — In a one-to-one IP Pool NAT configuration, the mapping is based on the subnet offset. The internal subnet 10.0.1.0/28 has 16 addresses (10.0.1.0–10.0.1.15), and the external pool 203.0.113.16/28 also has 16 addresses (203.0.113.16–203.0.113.31). The host 10.0.1.5 is the 6th usable IP (offset 5 from the network address), so it maps to the 6th IP in the external pool: 203.0.113.16 + 5 = 203.0.113.21.
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Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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