CCNA Network Infrastructure and Connectivity Practice Question
A host is configured with IP address 10.10.40.78/28. Which subnet contains that host?
⚠ Common exam trap
Avoid selecting a subnet range based on the nearest familiar-looking number; always calculate the correct block boundary.
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
✓
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.
Answer analysis
Option-by-option breakdown
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
A /28 subnet with network address .48 includes host addresses .49 through .62, with .63 as the broadcast address, so the usable range ends at .63. The host .78 lies well beyond this upper boundary, so it cannot belong to this subnet. While .48 is a valid network boundary, the block is too low in the address space. The correct subnet must have a range that extends at least to .78, which points to .64/28 instead.
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
With a /28 prefix length, the subnet mask is 255.255.255.240, and each subnet contains 16 addresses. The network address .64 starts the block .64–.79, where .64 is the network ID and .79 is the broadcast address. The host .78 is between these two, making it a valid host address within this subnet. Therefore, 10.10.40.64/28 is the correct network that contains .78.
- ✗
10.10.40.72/28
Why it's wrong here
A /28 subnet uses a 255.255.255.240 mask, giving a block size of 16 addresses. The network address must be a multiple of 16 in the fourth octet; .72 is not a multiple of 16, so it cannot be a subnet ID. In fact, .72 is a host address inside the 10.10.40.64/28 subnet, which spans .64 through .79. Therefore, .78 is not in a subnet starting at .72 because that subnet does not exist as a valid boundary.
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
The /28 block beginning at .80 covers addresses .80 through .95, with .80 as the network address and .95 as the broadcast address. Since the host address .78 is numerically less than .80, it falls in the preceding subnet, which is 10.10.40.64/28. Thus, 10.10.40.80/28 is the next subnet after the one containing .78, and cannot include it. This option incorrectly selects a subnet that starts 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
Why each answer is right or wrong
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.
✓10.10.40.64/28Correct answer▾
Why this is correct
With a /28 prefix length, the subnet mask is 255.255.255.240, and each subnet contains 16 addresses. The network address .64 starts the block .64–.79, where .64 is the network ID and .79 is the broadcast address. The host .78 is between these two, making it a valid host address within this subnet. Therefore, 10.10.40.64/28 is the correct network that contains .78.
✗10.10.40.48/28Wrong answer — click to see why▾
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.
✗10.10.40.72/28Wrong answer — click to see why▾
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.
✗10.10.40.80/28Wrong answer — click to see why▾
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?”
Visual reference
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Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
Prefix length
A prefix length is the number of bits in a subnet mask that identifies the network portion of an IP address, written after a slash (e.g., /24).
Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.