Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit
interface GigabitEthernet0/0 ip address 192.168.10.1 255.255.255.0 ip nat outside ! interface GigabitEthernet0/1 ip address 203.0.113.10 255.255.255.252 ip nat inside ! access-list 1 permit 192.168.10.0 0.0.0.255 ip nat inside source list 1 interface GigabitEthernet0/1 overload
Users on the inside network can browse the web, but return traffic is failing for some sessions. Based on the configuration, which change is required to make PAT work correctly?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
Apply ip nat enable on both interfaces.
`ip nat enable` is not the normal Cisco IOS syntax used for classic inside/outside NAT in this scenario. The directional role assignment is what needs correction.
B
Distractor review
Replace overload with pool.
Using a pool is another NAT design option, but it is not required for PAT to work. The current command already uses interface overload, which is valid. The failure is caused by reversed inside and outside roles.
C
Best answer
Swap the inside and outside NAT roles on the two interfaces.
This is correct because NAT depends on the router knowing which side is private and which side is public. The current configuration labels them the wrong way round. PAT with overload on the WAN interface is fine, but the interface roles must match the traffic direction.
D
Distractor review
Change access-list 1 to a standard ACL numbered 100.
Standard ACL 1 is already valid for identifying inside local addresses. There is no requirement to use ACL 100 for this kind of NAT rule. The bigger issue is that the interfaces are assigned the wrong NAT roles.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is confusing the NAT inside and outside interface roles. Candidates may see the private IP on an interface and mistakenly assign it as 'ip nat outside' or vice versa. This reverses the translation direction, causing return traffic to fail despite correct ACLs and overload commands. The trap exploits the assumption that the public IP must be inside or that the interface with the ACL is always inside. Understanding that NAT roles depend on network topology, not just IP addresses, is crucial to avoid this error.
Technical deep dive
How to think about this question
Network Address Translation (NAT) is a fundamental technology in Cisco networking that allows private IP addresses within an internal network to be translated into public IP addresses for communication over the internet. Port Address Translation (PAT), a form of dynamic NAT, enables multiple devices on a LAN to share a single public IP address by differentiating sessions using TCP/UDP port numbers. This is essential for conserving public IP addresses and enabling outbound internet access from private networks. For PAT to function correctly, the router must know which interfaces are inside (private) and which are outside (public). The 'ip nat inside' command marks interfaces connected to the private network, while 'ip nat outside' marks those connected to the public network. The NAT translation rule, such as 'ip nat inside source list 1 interface GigabitEthernet0/1 overload', uses an ACL to identify inside local addresses and translates them to the IP address of the outside interface. If these roles are reversed, the router cannot correctly translate and track sessions, causing return traffic to fail. A common exam trap is misconfiguring the NAT roles on interfaces, such as marking the private LAN interface as outside and the public WAN interface as inside. This reverses the translation logic and breaks PAT functionality. Practically, this causes outbound sessions to initiate but return traffic to be dropped, as the router cannot match return packets to existing NAT translations. Correctly assigning 'ip nat inside' to the LAN-facing interface and 'ip nat outside' to the WAN-facing interface resolves this issue and ensures proper PAT operation.
KKey Concepts to Remember
- NAT requires correct designation of inside and outside interfaces to properly translate private IP addresses to public IP addresses and vice versa.
- Port Address Translation (PAT) uses a single public IP address with port numbers to allow multiple inside hosts to share one outside IP address.
- The command 'ip nat inside source list <ACL> interface <interface> overload' enables PAT by translating inside local addresses to the IP of the specified outside interface.
- Access Control Lists (ACLs) define which inside local IP addresses are eligible for NAT translation by matching source IP addresses.
- Incorrectly assigning 'ip nat inside' and 'ip nat outside' to interfaces reverses the translation logic and causes return traffic to fail.
- The inside interface should face the private LAN subnet, while the outside interface should face the public or WAN network.
- Using 'overload' in NAT configuration enables PAT, allowing multiple sessions to share a single public IP address by differentiating ports.
- Standard ACLs such as ACL 1 can be used to identify inside local addresses for NAT, and changing ACL numbers does not fix interface role misconfiguration.
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.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
NAT requires correct designation of inside and outside interfaces to properly translate private IP addresses to public IP addresses and vice versa.
What is the correct answer to this question?
The correct answer is: Swap the inside and outside NAT roles on the two interfaces. — The problem is that the router has the NAT directions backwards. In simple terms, the interface facing the private LAN should be marked as inside, and the interface facing the public or WAN side should be marked as outside. Here, GigabitEthernet0/0 uses the private address 192.168.10.1, but it is configured as outside. GigabitEthernet0/1 uses the public address 203.0.113.10, but it is configured as inside. That reverses the translation logic and breaks normal PAT behavior. Technically, the command `ip nat inside source list 1 interface GigabitEthernet0/1 overload` is otherwise reasonable for dynamic PAT using the WAN interface address. The ACL also correctly identifies the inside local subnet. The essential fix is to mark G0/0 as `ip nat inside` and G0/1 as `ip nat outside`. Once the directions are corrected, PAT can create and track translations properly for outbound traffic and returning sessions.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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