CCNA Network Services and Security Practice Question
Exhibit
interface g0/0 ip address 192.168.20.1 255.255.255.0 ip nat outside ! interface g0/1 ip address 203.0.113.2 255.255.255.252 ip nat inside ! ip nat inside source list 10 interface g0/1 overload access-list 10 permit 192.168.20.0 0.0.0.255
The SVI for VLAN 20 has `ip nat outside` and the WAN interface has `ip nat inside`. Hosts in VLAN 20 must reach the internet through PAT, but users report no external connectivity. Which configuration issue best explains the problem?
⚠ Common exam trap
A frequent exam trap is assuming that the ACL or the subnet mask is the cause of NAT failure when the real issue is reversed inside and outside interface roles. Candidates often overlook the importance of interface designation commands (ip nat inside and ip nat outside), which are crucial for NAT operation. Without correct interface roles, the router cannot translate addresses properly, causing hosts to lose external connectivity even if ACLs and routing are correct. This trap is tempting because ACLs and subnetting are more familiar concepts, but interface roles are equally critical for NAT to function.
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
✓
The interfaces are marked with inside and outside in the wrong places
NAT overload works only when the inside and outside interfaces are identified correctly. Here the roles are reversed, so translations are not built in the right direction.
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 ACL should deny 192.168.20.0/24 instead of permit it
Why it's wrong here
A NAT ACL defines which inside local source addresses are eligible for translation, so it must explicitly permit the internal network that needs internet access. If you changed the ACL to deny 192.168.20.0/24, the router would exclude that subnet from translation and forward its traffic untranslated. Without translation, the private addresses are not routable on the public internet, so the connectivity goal fails. Permitting the subnet is the correct and necessary action for PAT to work.
When this WOULD be correct
In a different scenario where the question specifies that traffic from the 192.168.20.0/24 subnet should be blocked for security reasons, and the goal is to restrict access to certain external resources, then denying this subnet would be the correct answer.
- ✓
The interfaces are marked with inside and outside in the wrong places
Why this is correct
When PAT is configured, Cisco IOS identifies traffic to translate based on the inside and outside interface roles. If those labels are swapped, traffic arriving from the campus LAN appears on the 'outside' interface, so it is not considered an 'inside local' source and the translation rule does not trigger. As a result, packets are forwarded without translation and hosts on VLAN 20 cannot reach the internet through PAT. The fix is to mark the LAN-facing interface as 'inside' and the WAN-facing interface as 'outside' so the NAT process operates in the correct direction.
- ✗
PAT cannot be used with a /30 WAN link
Why it's wrong here
A /30 WAN subnet provides only two usable host addresses, but that is sufficient for PAT because the router needs just one public IP address for its outside interface or a NAT pool. PAT overload can translate many internal hosts to a single inside global address—whether that address is the interface IP or a pool entry—so the size of the WAN block does not limit it. In fact, /30 links are a common WAN deployment scenario where PAT is used to conserve scarce public IPv4 addresses.
When this WOULD be correct
In a different scenario, if a question states that a network is configured with a /30 WAN link and asks whether PAT can be implemented, the correct answer would be that PAT cannot be used due to the lack of available IP addresses for translation, making this option valid.
- ✗
NAT overload requires a route-map instead of an ACL
Why it's wrong here
Although a route-map can be used to select traffic for NAT overload, it is not a requirement—a standard ACL is perfectly valid for identifying the inside local networks to translate. The `ip nat inside source list <ACL> interface <WAN> overload` command uses an ACL to match source addresses, which is the classic and simplest PAT implementation. Route-maps are useful only when you need more granular matching or multiple NAT pools, but they are optional and not a prerequisite for PAT.
When this WOULD be correct
In a different scenario where the question specifies that advanced traffic management is needed for NAT overload, and the use of a route-map is explicitly required to match specific traffic types or conditions, this option would be correct. For example, if the question involved complex routing policies that necessitate route-maps for NAT configurations.
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.
✓The interfaces are marked with inside and outside in the wrong placesCorrect answer▾
Why this is correct
When PAT is configured, Cisco IOS identifies traffic to translate based on the inside and outside interface roles. If those labels are swapped, traffic arriving from the campus LAN appears on the 'outside' interface, so it is not considered an 'inside local' source and the translation rule does not trigger. As a result, packets are forwarded without translation and hosts on VLAN 20 cannot reach the internet through PAT. The fix is to mark the LAN-facing interface as 'inside' and the WAN-facing interface as 'outside' so the NAT process operates in the correct direction.
✗The ACL should deny 192.168.20.0/24 instead of permit itWrong answer — click to see why▾
Why this is wrong here
This option is incorrect because denying the subnet 192.168.20.0/24 would prevent any traffic from that VLAN from being processed, which is not the issue at hand. The problem lies in the configuration of inside and outside interfaces for NAT.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that traffic from the 192.168.20.0/24 subnet should be blocked for security reasons, and the goal is to restrict access to certain external resources, then denying this subnet would be the correct answer.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of ACL functionality, thinking that denying the VLAN subnet could be a straightforward solution to connectivity issues without fully analyzing the NAT configuration.
✗PAT cannot be used with a /30 WAN linkWrong answer — click to see why▾
Why this is wrong here
This option is incorrect because PAT can be used with any valid subnet, including a /30 WAN link, as long as there are sufficient IP addresses for translation. The issue in the question pertains to the incorrect marking of inside and outside interfaces, not the subnet size.
★ When this WOULD be the correct answer
In a different scenario, if a question states that a network is configured with a /30 WAN link and asks whether PAT can be implemented, the correct answer would be that PAT cannot be used due to the lack of available IP addresses for translation, making this option valid.
Why candidates choose this
Candidates may be tempted by this option due to a misunderstanding of NAT and PAT limitations, particularly in relation to subnet sizes and their implications for address translation, leading them to incorrectly assume a /30 subnet is incompatible with PAT.
✗NAT overload requires a route-map instead of an ACLWrong answer — click to see why▾
Why this is wrong here
NAT overload (PAT) does not require a route-map; it can be implemented using an ACL to define which internal addresses can be translated. Therefore, stating that NAT overload requires a route-map is incorrect in the context of this question about VLAN connectivity.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that advanced traffic management is needed for NAT overload, and the use of a route-map is explicitly required to match specific traffic types or conditions, this option would be correct. For example, if the question involved complex routing policies that necessitate route-maps for NAT configurations.
Why candidates choose this
Candidates may be tempted by this option due to confusion between basic NAT configurations and more advanced routing techniques, leading them to believe that a route-map is necessary for all forms of NAT, including PAT.
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?”
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Related to this question
Learn chapter
Configuring AAA with TACACS+ and RADIUS on IOS XE
Key term
NAT
NAT (Network Address Translation) is a method that allows multiple devices on a private network to share a single public IP address when accessing the internet.
Key term
SVI
A Switch Virtual Interface (SVI) is a logical, software-based interface on a multilayer switch that provides Layer 3 routing capabilities for a VLAN, allowing devices in different VLANs to communicate without an external router.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.