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220-1201 Practice Question: A small office has a single public IP address and…
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?
⚠ Common exam trap
Many exam-takers 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.
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.
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 (Domain Name System) translates human-readable domain names into numerical IP addresses. If DNS resolution were failing intermittently or completely, users would experience "DNS lookup failed" errors or inability to reach *any* website by name, not just some, because the initial name-to-IP translation would fail universally. This scenario describes a more selective failure, not a global DNS issue affecting all web access.
- ✗
DHCP
Why it's wrong here
DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses and other network configuration parameters to devices joining a network. DHCP exhaustion would primarily manifest as new devices being unable to obtain an IP address and connect to the network at all, rather than existing connected devices experiencing intermittent slowness or timeouts to specific external websites. It does not directly impact the ability of already-configured devices to access external resources.
- ✓
NAT
Why this is correct
Network Address Translation (NAT) allows multiple private IP addresses within a local network to share a single public IP address for outbound internet access. When the NAT device's translation table, which maps internal private IP/port combinations to external public IP/port combinations, becomes exhausted, it cannot create new entries for subsequent outbound connection requests. This leads to new connections timing out or failing for some websites while existing or less demanding connections might still function, perfectly matching the "intermittent slowness and timeouts to some sites" symptom.
- ✗
ICMP
Why it's wrong here
ICMP (Internet Control Message Protocol) is primarily used for network diagnostic and error-reporting functions, such as the `ping` and `traceroute` utilities. While ICMP messages can indicate network problems, the protocol itself does not directly facilitate web browsing or other application-layer data transfer. Therefore, issues with ICMP would not directly cause intermittent slowness or timeouts when accessing specific websites, as web traffic relies on TCP/IP.
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.
Go deeper
Related to this question
Learn chapter
Internet Connection Types
Key term
Public IP address
A globally unique IP address assigned to a device that allows it to communicate directly over the internet.
Key term
NAT overload
NAT overload is a form of network address translation that allows many devices on a private network to share a single public IP address by using unique port numbers to track each connection.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1201 practice question is part of Courseiva's free CompTIA 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 220-1201 exam.