CCNA Network Services and Security Practice Question
A network administrator configures PAT on a router to allow internal hosts in the 10.10.10.0/24 subnet to access the Internet. Afterward, users report that they can ping public IP addresses but cannot access any websites. The administrator verifies that the access list for NAT matches the correct subnet, and the 'ip nat inside source list 1 interface GigabitEthernet0/1 overload' command is applied. What is the most likely cause of this issue?
⚠ Common exam trap
Cisco often tests the difference between IP connectivity and DNS resolution. Candidates may assume that PAT exhaustion is the cause when ping works but websites fail; however, DNS is the culprit because ping uses IP addresses and does not need DNS.
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 router's DNS proxy is misconfigured, preventing resolution of website names.
The ability to ping public IP addresses confirms that IP routing and PAT are working for ICMP traffic. However, the failure to access websites by name while ping works indicates a DNS resolution problem. If the PAT translation table were full, new ICMP echo requests would also be dropped because they require new NAT translations. Therefore, the most likely cause is a misconfigured DNS proxy or DNS server, preventing name resolution for HTTP/HTTPS sessions.
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 router's DNS proxy is misconfigured, preventing resolution of website names.
Why this is correct
A misconfigured DNS proxy would prevent the router from resolving domain names into IP addresses, so users would still be able to communicate with hosts by IP. However, the symptom specifically states that users can successfully ping public IP addresses, which proves IP connectivity and routing are working. Web browsing separately requires TCP port 80/443 connections, and PAT exhaustion drops new TCP sessions while still permitting ICMP echo requests, which do not need a port mapping. Therefore, DNS is not the root cause; the failure is isolated to TCP connection establishment.
- ✗
The PAT translation table is full, causing new TCP connection requests to be dropped.
Why it's wrong here
PAT uses source port translation to map many internal addresses to a single public IP. ICMP (ping) does not consume a port mapping and can still be translated even when the table is exhausted. New TCP connections, required for web traffic, will fail when no free source port is available.
When this WOULD be correct
When users can ping IPs but not browse the web, and PAT is correctly configured, port exhaustion is the most plausible cause.
- ✗
The 'overload' keyword was omitted, causing the router to use dynamic NAT with a single-address pool.
Why it's wrong here
The configuration shown includes the 'overload' keyword. Even if it were missing, dynamic NAT without overload would not allow any internal host other than the first one to use the IP, but pings would also fail from other hosts. This does not match the symptom.
- ✗
A static NAT entry for a web server is using the same public IP address as the PAT overload.
Why it's wrong here
Configuring a static NAT entry with the same IP as the overloaded outside interface would typically cause a conflict or prevent the PAT translation from working correctly for all traffic, including ping. The router would likely generate an error or reject the command.
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 router's DNS proxy is misconfigured, preventing resolution of website names.Correct answer▾
Why this is correct
A misconfigured DNS proxy would prevent the router from resolving domain names into IP addresses, so users would still be able to communicate with hosts by IP. However, the symptom specifically states that users can successfully ping public IP addresses, which proves IP connectivity and routing are working. Web browsing separately requires TCP port 80/443 connections, and PAT exhaustion drops new TCP sessions while still permitting ICMP echo requests, which do not need a port mapping. Therefore, DNS is not the root cause; the failure is isolated to TCP connection establishment.
✗The PAT translation table is full, causing new TCP connection requests to be dropped.Wrong answer — click to see why▾
Why this is wrong here
PAT uses source port translation to map many internal addresses to a single public IP. ICMP (ping) does not consume a port mapping and can still be translated even when the table is exhausted. New TCP connections, required for web traffic, will fail when no free source port is available.
★ When this WOULD be the correct answer
When users can ping IPs but not browse the web, and PAT is correctly configured, port exhaustion is the most plausible cause.
✗The 'overload' keyword was omitted, causing the router to use dynamic NAT with a single-address pool.Wrong answer — click to see why▾
Why this is wrong here
The command output explicitly shows 'overload', and the symptom (ping works, TCP fails) is inconsistent with a missing overload keyword.
✗A static NAT entry for a web server is using the same public IP address as the PAT overload.Wrong answer — click to see why▾
Why this is wrong here
A static NAT conflict would affect all traffic (including ICMP), not just web traffic.
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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Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
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.
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JA
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.