Question 393 of 1,020

IP Addressing →mediumMultiple ChoiceObjective-mapped

How to Subnet a /16 Network into 8 Subnets with 1000 Hosts

This 220-1201 practice question tests your understanding of ip addressing. 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 company has a network with the IP range 172.16.0.0/16. They need to create 8 separate subnets for different departments, each with at least 1000 usable hosts. What subnet mask should be used?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.

A
255.255.240.0
Why wrong: This is a /20 subnet mask, which would provide 16 subnets (2^4) but only 4094 usable hosts per subnet, which is sufficient but not the most efficient for exactly 8 subnets.
B
255.255.224.0
A /19 mask (255.255.224.0) provides 8 subnets (2^3) with 8190 usable hosts each, meeting both requirements.
C
255.255.248.0
Why wrong: This is a /21 subnet mask, which would provide 32 subnets (2^5) but only 2046 usable hosts per subnet, which is enough but not the best fit for exactly 8 subnets.
D
255.255.192.0
Why wrong: This is a /18 subnet mask, which would provide 4 subnets (2^2), not enough to create 8 separate subnets.

Full breakdown with real-world context →

Quick Answer

The correct subnet mask is 255.255.224.0. This is because to create 8 subnets from a /16 network, you must borrow 3 host bits for subnetting, as 2³ equals 8. Borrowing those 3 bits extends the network prefix from /16 to /19, which in dotted decimal is 255.255.224.0. Each of these /19 subnets then provides 2¹³ minus 2, or 8190 usable hosts, easily satisfying the requirement of at least 1000 hosts per subnet. On the CompTIA A+ Core 1 220-1201 exam, this type of question tests your ability to apply subnetting fundamentals under real-world constraints—specifically, balancing the number of subnets against host capacity. A common trap is to stop at the subnet count without verifying the host count, or to mistakenly use a /24 mask which would only yield 254 hosts per subnet. A helpful memory tip: remember that borrowing 3 bits from a /16 gives you a /19, and the magic number for the third octet is 32 (256 minus 224), so your subnets increment by 32 in that octet.

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

✓

255.255.224.0

To support 8 subnets with at least 1000 usable hosts each, you need to borrow enough bits from the host portion to create 8 subnets (2^3 = 8) while leaving at least 10 host bits (2^10 - 2 = 1022 usable hosts). Starting from /16, borrowing 3 bits gives a /19 prefix, which translates to subnet mask 255.255.224.0. This mask provides exactly 8 subnets and 8190 total hosts per subnet (2^13 - 2 = 8190), well exceeding the 1000-host requirement.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
255.255.240.0
Why it's wrong here
This is a /20 subnet mask, which would provide 16 subnets (2^4) but only 4094 usable hosts per subnet, which is sufficient but not the most efficient for exactly 8 subnets.
✓
255.255.224.0
Why this is correct
A /19 mask (255.255.224.0) provides 8 subnets (2^3) with 8190 usable hosts each, meeting both requirements.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
255.255.248.0
Why it's wrong here
This is a /21 subnet mask, which would provide 32 subnets (2^5) but only 2046 usable hosts per subnet, which is enough but not the best fit for exactly 8 subnets.
✗
255.255.192.0
Why it's wrong here
This is a /18 subnet mask, which would provide 4 subnets (2^2), not enough to create 8 separate subnets.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that you need to borrow the minimum number of bits to satisfy the host requirement first, leading candidates to pick a mask that provides exactly 1000 hosts (like /22 or /23) but fails to provide enough subnets, or they mistakenly use the formula 2^n - 2 for subnets instead of 2^n, causing them to borrow an extra bit and select a mask like 255.255.240.0.

Detailed technical explanation

How to think about this question

The subnet mask 255.255.224.0 corresponds to a /19 CIDR prefix, where the third octet in binary is 11100000. This means the network portion uses 19 bits, leaving 13 bits for hosts. The formula 2^(number of host bits) - 2 yields 8190 usable addresses per subnet, which is far more than the 1000 required, but the key constraint is the subnet count: 2^(borrowed bits) must be at least 8, and borrowing exactly 3 bits (from /16 to /19) yields exactly 8 subnets. In real-world scenarios, network engineers often use Variable Length Subnet Masking (VLSM) to further subnet these /19 blocks into smaller subnets for different departments, but the question asks for a fixed mask that meets both the subnet count and host count simultaneously.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 220-1201 question test?

IP Addressing — This question tests IP Addressing — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: 255.255.224.0 — To support 8 subnets with at least 1000 usable hosts each, you need to borrow enough bits from the host portion to create 8 subnets (2^3 = 8) while leaving at least 10 host bits (2^10 - 2 = 1022 usable hosts). Starting from /16, borrowing 3 bits gives a /19 prefix, which translates to subnet mask 255.255.224.0. This mask provides exactly 8 subnets and 8190 total hosts per subnet (2^13 - 2 = 8190), well exceeding the 1000-host requirement.

What should I do if I get this 220-1201 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "least". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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