Quick Answer
The correct subnet is 10.10.40.64/28 because the host IP 10.10.40.78 falls within the 64–79 range of the fourth octet, which is determined by the /28 prefix length’s block size of 16. A /28 subnet divides the octet into 16-address blocks starting at multiples of 16, so the network address is the first address of the block containing the host—here, 64. On the CCNA 200-301 v2 exam, this type of question tests your ability to convert a prefix length into a block size and then locate the correct subnet boundary, a skill essential for IP addressing and routing. A common trap is picking a familiar number like 40 or 78 instead of calculating the block start; remember that the network address is always a multiple of the block size. For a quick memory tip, think “block size is 2^(32-prefix), then find the multiple just below the host octet.”
This 200-301 practice question tests your understanding of network infrastructure and connectivity. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. A key principle to apply: a /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses.. 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 host is configured with IP address 10.10.40.78/28. Which subnet contains that host?
10.10.40.48/28
Why wrong: This is wrong because that block ends at .63.
10.10.40.64/28
This is correct because .78 falls inside the .64 through .79 range.
10.10.40.72/28
Why wrong: This is wrong because /28 boundaries do not start at .72.
10.10.40.80/28
Why wrong: This is wrong because that block begins above the host address.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
10.10.40.64/28
A /28 subnet has a block size of 16. In simple terms, the fourth-octet ranges are 0–15, 16–31, 32–47, 48–63, 64–79, 80–95, and so on. Because 78 falls inside the 64–79 block, the network address for this host’s subnet is 10.10.40.64/28. This style of subnetting question checks whether you can move from prefix length to block size and then place the host into the correct range. The common mistake is choosing the nearest familiar-looking number instead of the actual block boundary.
Key principle: A /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
10.10.40.48/28
Why it's wrong here
This is wrong because that block ends at .63.
When this WOULD be correct
If the question asked for the subnet that includes the IP address 10.10.40.50, then option A (10.10.40.48/28) would be correct, as it would encompass the range of IPs from 10.10.40.48 to 10.10.40.63, including 10.10.40.50.
10.10.40.64/28
Why this is correct
This is correct because .78 falls inside the .64 through .79 range.
Related concept
A /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses.
10.10.40.72/28
Why it's wrong here
This is wrong because /28 boundaries do not start at .72.
When this WOULD be correct
If the exam question asked for the subnet that includes the IP address 10.10.40.72, then option C would be correct, as it would represent the subnet containing that specific address.
10.10.40.80/28
Why it's wrong here
This is wrong because that block begins above the host address.
When this WOULD be correct
If the question asked which subnet contains the IP address 10.10.40.80, then option D would be correct, as 10.10.40.80/28 encompasses the range from 10.10.40.80 to 10.10.40.95.
Option-by-option analysis
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
Why this is correct
This is correct because .78 falls inside the .64 through .79 range.
Why this is wrong here
The subnet 10.10.40.48/28 includes addresses 10.10.40.48 through 10.10.40.63. The host address 10.10.40.78 is outside this range, so it does not belong to this subnet.
★ When this WOULD be the correct answer
If the question asked for the subnet that includes the IP address 10.10.40.50, then option A (10.10.40.48/28) would be correct, as it would encompass the range of IPs from 10.10.40.48 to 10.10.40.63, including 10.10.40.50.
Why candidates choose this
Students might mistakenly think that because 78 is close to 48, it could be in the same subnet, but they overlook the subnet boundaries defined by the /28 prefix length.
Why this is wrong here
The subnet 10.10.40.72/28 is not a valid subnet because /28 subnets have boundaries that are multiples of 16. The valid subnet starting addresses for /28 are 0, 16, 32, 48, 64, 80, etc. 72 is not a multiple of 16, so this is not a valid network address.
★ When this WOULD be the correct answer
If the exam question asked for the subnet that includes the IP address 10.10.40.72, then option C would be correct, as it would represent the subnet containing that specific address.
Why candidates choose this
Students might think that any address can be a subnet start, especially if they see that 72 is close to 78, but they forget that subnet boundaries must align with the prefix length.
Why this is wrong here
The subnet 10.10.40.80/28 includes addresses 10.10.40.80 through 10.10.40.95. The host address 10.10.40.78 is below this range, so it does not belong to this subnet.
★ When this WOULD be the correct answer
If the question asked which subnet contains the IP address 10.10.40.80, then option D would be correct, as 10.10.40.80/28 encompasses the range from 10.10.40.80 to 10.10.40.95.
Why candidates choose this
Students might confuse the host address with the network address or think that 78 is close to 80, but they fail to recognize that the subnet starts at 80, not 78.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Avoid selecting a subnet range based on the nearest familiar-looking number; always calculate the correct block boundary.
Detailed technical explanation
Subnetting is a fundamental concept in IP networking that divides a larger network into smaller, manageable subnetworks. A /28 subnet mask corresponds to 255.255.255.240, which means the last octet has 4 bits for host addressing, allowing for 16 IP addresses per subnet block. These addresses include the network address, usable host addresses, and the broadcast address. Understanding how to calculate subnet boundaries is critical for efficient IP address allocation and routing. To determine the subnet containing a specific IP address like 10.10.40.78/28, you calculate the block size by subtracting the subnet mask from 256, which yields 16. The subnets increment in blocks of 16 in the last octet: 0-15, 16-31, 32-47, 48-63, 64-79, and so forth. Since 78 falls between 64 and 79, the correct subnet is 10.10.40.64/28. This method ensures precise identification of the network address and prevents misconfiguration. A common exam trap is confusing subnet boundaries with arbitrary numbers that appear close to the host IP, such as 10.10.40.72/28, which is invalid because /28 subnets do not start at .72. Cisco exams test your ability to calculate subnet ranges accurately rather than relying on memorized or approximate values. Practically, correct subnetting ensures proper routing and prevents IP conflicts in a network environment.
Key takeaway
A /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses.
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 a /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — A /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses..
The correct answer is: 10.10.40.64/28 — A /28 subnet has a block size of 16. In simple terms, the fourth-octet ranges are 0–15, 16–31, 32–47, 48–63, 64–79, 80–95, and so on. Because 78 falls inside the 64–79 block, the network address for this host’s subnet is 10.10.40.64/28. This style of subnetting question checks whether you can move from prefix length to block size and then place the host into the correct range. The common mistake is choosing the nearest familiar-looking number instead of the actual block boundary.
Review a /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses., then practise related 200-301 questions on the same topic to reinforce the concept.
A /28 subnet mask creates subnets with 16 IP addresses each, including network and broadcast addresses.
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A host is configured with IP address 192.168.50.94/27. Which subnet contains that host?
hardWhy B: A /27 subnet has a block size of 32. In simple terms, the fourth-octet ranges are 0–31, 32–63, 64–95, 96–127, and so on. Because 94 falls inside the 64–95 range, the network address for the host’s subnet is 192.168.50.64/27. This kind of question tests whether you can move from prefix length to block size and then place the host inside the correct interval. The most common mistake is choosing a nearby boundary like 96 or 32 without calculating the actual block that contains the address.
Variation 2. A host has the address 10.10.10.94/27. Which subnet contains that host?
hardWhy B: A /27 mask creates subnets in blocks of 32 addresses. In plain language, that means the fourth-octet ranges are 0–31, 32–63, 64–95, 96–127, and so on. Since the host address ends in 94, it falls inside the 64–95 block. That means the subnet is 10.10.10.64/27. This is a classic subnetting task because it checks whether you can move from prefix length to block size and then locate the host inside the correct range. The key skill is recognizing the increment boundary and not guessing based only on the nearest familiar address.
Variation 3. A host address is 192.168.22.145/28. Which subnet contains that host?
hardWhy B: A /28 prefix creates address blocks of 16. In practical terms, the fourth-octet ranges are 0–15, 16–31, 32–47, and so on. Because 145 falls inside the 144–159 range, the network address of the containing subnet is 192.168.22.144/28. This type of question checks whether you can move from prefix length to block size and then place a host into the correct interval. The common mistake is choosing a nearby familiar number instead of calculating the actual block boundary.
Last reviewed: May 17, 2026
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