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220-1201 Practice Question: A company has a network with the IP range…
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?
⚠ Common exam trap
The 220-1201 exam 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.
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.
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 subnet mask corresponds to a /20 CIDR prefix. If applied to a Class B network (e.g., /16), it borrows 4 bits for subnetting, resulting in 2^4 = 16 available subnets. While this provides more than the required 8 subnets, it leaves only 12 host bits, allowing for 2^12 - 2 = 4094 usable hosts per subnet. This is less efficient and provides fewer hosts than the optimal solution, which typically aims for the minimum number of subnets to meet the requirement while maximizing host capacity.
- ✓
255.255.224.0
Why this is correct
This subnet mask, represented as a /19 CIDR prefix, is the correct choice. It borrows 3 bits from the host portion of a Class B network address (e.g., /16) for subnetting, yielding 2^3 = 8 distinct subnets. The remaining 13 host bits provide 2^13 - 2 = 8190 usable host addresses per subnet. This configuration precisely meets the requirement for 8 subnets while offering a substantial number of hosts for each.
- ✗
255.255.248.0
Why it's wrong here
This subnet mask translates to a /21 CIDR prefix. Utilizing 5 bits for subnetting (e.g., from a /16 network) would create 2^5 = 32 subnets, which is significantly more than the 8 subnets required. Furthermore, with only 11 host bits remaining, each subnet would support 2^11 - 2 = 2046 usable hosts. This provides an excessive number of subnets and a considerably lower host count per subnet compared to the optimal solution.
- ✗
255.255.192.0
Why it's wrong here
This subnet mask corresponds to a /18 CIDR prefix. When applied to a Class B network (e.g., /16), it borrows only 2 bits for subnetting. This configuration would result in 2^2 = 4 available subnets, which is insufficient to meet the requirement of creating 8 separate subnets. Although it would provide a large number of hosts (2^14 - 2 = 16382) per subnet, the primary condition of having enough subnets is not satisfied.
Visual reference
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Related to this question
Learn chapter
Network Configuration: IP, DNS, DHCP
Key term
Subnet mask
A subnet mask is a 32-bit number that helps a computer or network device determine which part of an IP address identifies the network and which part identifies the host device on that network.
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using subnet masks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1201 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 220-1201 exam.