- A
DNS
Why wrong: DNS resolves domain names; if it were failing, all websites would be affected, not just some.
- B
DHCP
Why wrong: DHCP assigns IP addresses; exhaustion would prevent new devices from connecting, not cause intermittent slowness.
- C
NAT
NAT table exhaustion prevents new outbound connections, causing timeouts for some sites while others work.
- D
ICMP
Why wrong: ICMP is used for diagnostics like ping; it does not affect web browsing directly.
Question 780 of 1,020
NAT Table Exhaustion: Troubleshooting for CompTIA A+ 220-1201
This 220-1201 practice question tests your understanding of network protocols. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A small office has a single public IP address and uses a router to provide internet access to 20 devices. Users report that some websites load slowly or time out, while others work fine. The router's NAT table shows many entries. Which protocol is most likely being exhausted, causing these intermittent issues?
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.
Quick Answer
The answer is NAT table exhaustion, specifically the depletion of available ports in the router’s Network Address Translation table. When a small office with a single public IP address supports 20 devices, each outbound connection consumes a unique port number in the NAT table; once the table fills up—often with thousands of simultaneous entries from web browsing or streaming—the router cannot map new private IP requests to the public IP, causing intermittent timeouts and slow loads for some sites while others work. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your understanding of how NAT resources become a bottleneck in small networks, and a common trap is confusing this with a DNS or bandwidth issue—remember, if the router’s NAT table shows many entries, the problem is port exhaustion, not a lack of IP addresses. Memory tip: think of the NAT table like a parking lot with a limited number of spaces—once every spot is taken, new cars (connections) must wait or are turned away.
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
✓
NAT
The router's NAT table has many entries, indicating that the single public IP address is being heavily used to translate private IPs for 20 devices. When the NAT table exhausts its available port mappings (typically 65,535 per public IP, but often limited by router resources), new outbound connections cannot be established, causing some websites to time out while existing connections continue to work. This is a classic symptom of NAT overload (PAT) exhaustion.
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.
- ✗
DNS
Why it's wrong here
DNS resolves domain names; if it were failing, all websites would be affected, not just some.
- ✗
DHCP
Why it's wrong here
DHCP assigns IP addresses; exhaustion would prevent new devices from connecting, not cause intermittent slowness.
- ✓
NAT
Why this is correct
NAT table exhaustion prevents new outbound connections, causing timeouts for some sites while others work.
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.
- ✗
ICMP
Why it's wrong here
ICMP is used for diagnostics like ping; it does not affect web browsing directly.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse NAT exhaustion with DNS failure, but DNS would cause all sites to fail, whereas NAT exhaustion causes intermittent failures as the table fills and empties.
Detailed technical explanation
How to think about this question
Under the hood, NAT overload (Port Address Translation) maps multiple private IPs to a single public IP by assigning unique source port numbers. The router tracks these in a NAT table using the 5-tuple (source IP, source port, destination IP, destination port, protocol). When the table is full (e.g., 65,535 entries per public IP), the router cannot create new mappings, causing TCP SYN packets to be dropped. In real-world scenarios, this often occurs with P2P applications or many simultaneous connections, and can be verified with 'show ip nat translations' on Cisco routers.
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.
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
What to study next
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Network Protocols — This question tests Network Protocols — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: NAT — The router's NAT table has many entries, indicating that the single public IP address is being heavily used to translate private IPs for 20 devices. When the NAT table exhausts its available port mappings (typically 65,535 per public IP, but often limited by router resources), new outbound connections cannot be established, causing some websites to time out while existing connections continue to work. This is a classic symptom of NAT overload (PAT) exhaustion.
What should I do if I get this 220-1201 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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Last reviewed: Jul 4, 2026
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