Subnet Mask Calculation for Host Capacity vs Subnet Count
Short answer
B Use 255.255.252.0 (/22) to provide enough addresses for 1,000 hosts per subnet, as it is the only option that meets the host capacity requirement.
Question
Key Takeaways
When subnet count and host count constraints conflict, prioritize the host count requirement as it defines the functional limit of the subnet.
Learn how to calculate the correct subnet mask when balancing subnet count and host requirements. The correct answer is B (255.255.252.0) because it supports over 1,000 hosts.
Common mistake: Choosing A (255.255.0.0) because it is the default Class B mask, ignoring the need to divide the network. Or choosing C/D because they create more subnets but fail to support the required 1,000 hosts.
Expert Analysis
The question requires dividing a Class B network (default mask /16 or 255.255.0.0) into four equal subnets, each supporting at least 1,000 hosts.
First, determine the host bits required. The formula for usable hosts is 2^h - 2. To support 1,000 hosts, we need 2^h >= 1,002. Since 2^9 = 512 and 2^10 = 1,024, we require 10 host bits. A subnet with 10 host bits has a prefix length of 32 - 10 = /22.
Second, verify the subnetting requirement. A Class B network has a default /16 prefix. To create four equal subnets, we need to borrow 2 bits from the host portion (since 2^2 = 4). Borrowing 2 bits from /16 results in a /18 prefix (16 + 2 = 18).
There is a conflict between the 'four equal subnets' constraint (/18) and the '1,000 hosts per subnet' constraint (/22). Let's evaluate the options:
- A (/16): Supports ~65k hosts, but creates only 1 subnet (no division).
- B (/22): Supports 1,022 hosts (meets the 1,000 host requirement). It divides the /16 space into 2^(22-16) = 2^6 = 64 subnets. This exceeds the 'four equal subnets' minimum but satisfies the critical host capacity constraint which the others fail.
- C (/24): Supports 254 hosts. Fails the host requirement.
- D (/25): Supports 126 hosts. Fails the host requirement.
In certification exams, if constraints conflict, the host capacity is often the harder limit because you cannot reduce hosts below the requirement without breaking functionality. Option B is the only mask that allows for 1,000 hosts. While it creates more than four subnets, it is the only viable solution among the choices. The comment suggesting B correctly identifies the /22 prefix derived from the host bit calculation.
Exam Strategy
Always calculate the minimum host bits first using 2^h - 2 >= required hosts. Then check if the resulting prefix length allows for the required number of subnets. If the constraints conflict, select the mask that satisfies the most critical operational requirement, typically host capacity, as listed in the options.
Frequently Asked Questions
Why not use a /18 mask for four subnets?
What does 255.255.252.0 represent in binary?
Discussion 3 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 3
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AnderSpain 👍 1 Answered BProblem requirements: A Class B network (which by default uses a /16 mask or 255.255.0.0) needs to be divided into four equal subnets, where each subnet must accommodate a range of 1,000 hosts. TXT+ 1 Calculating host bits: To accommodate at least 1,000 hosts per subnet, we need to calculate how many host bits are required using the formula 2^h - 2. With 10 bits (2^10 = 1024 total addresses, 1,024 - 2 = 1,022 usable hosts), we exactly meet the 1,000 hosts required. TXT+ 1 Subnet mask calculation: An IPv4 address has a total of 32 bits. TXT If we reserve 10 bits for the hosts, we are left with 32−10=22 bits for the network (i.e., a /22 prefix). TXT A subnet mask with a /22 prefix in dotted decimal format is written as 255.255.252.0 (since the first 22 bits are set to 1). TXT
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ERB14 👍 1 Answered BThe correct answer is B. 255.255.252.0. Determine the Subnet Mask Prefix: An IPv4 address contains 32 total bits. Subtracting the 10 host bits gives the network prefix length: $$32 - 10 = /22\text{ prefix length}$$ Convert /22 to Dotted-Decimal Notation:A /22 subnet mask has 22 consecutive ones in binary:11111111.11111111.11111100.00000000 Converted to decimal: 255.255.252.0
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Skri113xX 👍 1 Answered AIf I had to guess, I’d pick A based on the wording of the question.
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