Quick Answer
The answer is UDP port 67. This is the correct choice because the Dynamic Host Configuration Protocol (DHCP) server listens on UDP port 67 to receive client requests and send configuration data, while the client uses UDP port 68. When a firewall blocks port 67, the server cannot hear the DHCPDISCOVER broadcasts from clients, preventing IP address assignment entirely. On the CompTIA A+ Core 1 220-1201 exam, this concept tests your understanding of core network services and port numbers, often appearing in troubleshooting scenarios where clients fail to obtain an IP address. A common trap is confusing the server port (67) with the client port (68) or assuming DHCP uses TCP instead of UDP. To remember, think of the server as the “senior” port—67 is higher than 68, just as the server is above the client in the hierarchy.
This 220-1201 practice question tests your understanding of tcp & udp ports. 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 DHCP issue where clients are not receiving IP addresses. Which port must be open on the firewall for DHCP server communication?
67
Port 67 is the DHCP server port, used to receive client requests and send offers.
68
Why wrong: Port 68 is the DHCP client port, used by clients to send requests, but the question asks for server communication.
53
Why wrong: Port 53 is DNS, not DHCP.
123
Why wrong: Port 123 is NTP, used for time synchronization, not DHCP.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
67
DHCP uses UDP ports 67 (server) and 68 (client). Opening port 67 allows DHCP server broadcasts to reach clients.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
67
68
Why it's wrong here
Port 68 is the DHCP client port, used by clients to send requests, but the question asks for server communication.
53
123
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 220-1201 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
TCP & UDP Ports — This question tests TCP & UDP Ports — CIDR notation defines the prefix length..
The correct answer is: 67 — DHCP uses UDP ports 67 (server) and 68 (client). Opening port 67 allows DHCP server broadcasts to reach clients.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 220-1201 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Last reviewed: Jun 18, 2026
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