The answer is to inspect and clean the fiber connector. A red LOS light on the ONT means the device is detecting no optical signal, and even if the fiber cable looks intact from the outside, microscopic dust, oil, or scratches on the connector end-face are the most frequent culprits for signal loss. This scenario directly tests your understanding of fiber optic physical layer troubleshooting for the CompTIA A+ Core 1 220-1201 exam, where a common trap is assuming a visually intact cable is fully functional—the real issue is often contamination at the termination point. A useful memory tip is “Red LOS, clean the gloss,” reminding you that a loss of signal almost always starts with a dirty connector before you suspect a broken cable.
220-1101 Internet Connection Types Practice Question
This 220-1201 practice question tests your understanding of internet connection types. 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 technician is troubleshooting a fiber optic connection where the ONT (Optical Network Terminal) has a red 'LOS' (Loss of Signal) light. The fiber cable appears intact. What should the technician check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inspect and clean the fiber connector.
A red LOS light indicates no optical signal is reaching the ONT. Even if the cable looks intact, the most common cause is a dirty or damaged connector at the termination point. Cleaning the fiber end with a proper kit often resolves the issue.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Replace the ONT immediately.
Why it's wrong here
Replacing the ONT is premature; the issue is likely with the physical connection or signal, not the ONT itself.
✗
Check the power supply to the ONT.
Why it's wrong here
If the LOS light is on, the ONT has power; a power issue would show no lights at all.
✓
Inspect and clean the fiber connector.
Why this is correct
Dirty or damaged connectors are a common cause of signal loss in fiber; cleaning often restores the link.
Related concept
Static NAT maps one inside address to one outside address.
✗
Reboot the router connected to the ONT.
Why it's wrong here
Rebooting the router will not fix a missing optical signal; the LOS light indicates a physical layer issue upstream.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
If the LOS light is on, the ONT has power; a power issue would show no lights at all.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 220-1201 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 220-1201 NAT questions on configuration and troubleshooting.
Internet Connection Types — This question tests Internet Connection Types — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Inspect and clean the fiber connector. — A red LOS light indicates no optical signal is reaching the ONT. Even if the cable looks intact, the most common cause is a dirty or damaged connector at the termination point. Cleaning the fiber end with a proper kit often resolves the issue.
What should I do if I get this 220-1201 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 220-1201 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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