Quick Answer
The answer is DHCP, as cable modems typically use this protocol to obtain a public IP address from the ISP. When the modem powers on, it sends a DHCPDISCOVER broadcast across the cable network, and the ISP’s DHCP server replies with an IP address, subnet mask, default gateway, and DNS server information, enabling internet access. On the CompTIA ITF+ FC0-U61 exam, this concept tests your understanding of how customer premises equipment (CPE) like cable modems dynamically receive their public-facing addresses, often contrasting with static assignment or PPPoE in DSL networks. A common trap is confusing cable modem DHCP with the DHCP used inside a local router for private IPs—remember, the modem itself is a DHCP client, not a server. For a memory tip, think “Modem as Client”: the modem discovers, the ISP assigns, and the internet flows.
This FC0-U61 practice question tests your understanding of infrastructure. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 business purchases a cable modem from the ISP. They need to obtain a public IP address to access the internet. Which method does the cable modem typically use?
NAT
Why wrong: NAT is used to translate private addresses to public, not to obtain an IP.
DHCP
Cable modems use DHCP to obtain an IP address from the ISP automatically.
Static assignment
Why wrong: Static assignment is not typical for cable modems; ISPs use DHCP.
APIPA
Why wrong: APIPA is used when DHCP fails, resulting in a private address (169.254.x.x).
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
DHCP
Cable modems typically use DHCP to obtain a public IP address from the ISP. When the modem powers on, it sends a DHCPDISCOVER broadcast, and the ISP's DHCP server responds with an IP address, subnet mask, default gateway, and DNS server information. This is the standard method for dynamically assigning public IP addresses to customer premises equipment (CPE) in cable networks.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
NAT
Why it's wrong here
NAT is used to translate private addresses to public, not to obtain an IP.
DHCP
Why this is correct
Cable modems use DHCP to obtain an IP address from the ISP automatically.
Related concept
Read the scenario before looking for a memorised answer.
Static assignment
Why it's wrong here
Static assignment is not typical for cable modems; ISPs use DHCP.
APIPA
Why it's wrong here
APIPA is used when DHCP fails, resulting in a private address (169.254.x.x).
Common exam traps
The trap here is that candidates often confuse the cable modem's method of obtaining an IP address with the router's NAT function, leading them to incorrectly select NAT as the answer when DHCP is the correct mechanism used by the modem to request a public IP from the ISP.
Detailed technical explanation
The cable modem communicates with the ISP's DHCP server using the DOCSIS (Data Over Cable Service Interface Specification) protocol, which includes DHCP options like Option 82 (Relay Agent Information) to identify the modem's CMAC address and ensure proper IP assignment. In many cable networks, the modem receives a private IP address from the ISP's DHCP server for management purposes, while the public IP is assigned to the router behind the modem via DHCP. This dual-DHCP process is a subtle behavior that can confuse candidates who assume the modem itself always gets the public IP.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
The correct answer is: DHCP — Cable modems typically use DHCP to obtain a public IP address from the ISP. When the modem powers on, it sends a DHCPDISCOVER broadcast, and the ISP's DHCP server responds with an IP address, subnet mask, default gateway, and DNS server information. This is the standard method for dynamically assigning public IP addresses to customer premises equipment (CPE) in cable networks.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This FC0-U61 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 FC0-U61 exam.
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