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Difficulty: MediumIPv4 Addressing and Subnetting

A network architect is assigned the IPv4 address block 10.116.0.0/1810.116.0.0/18 to provision subnets for remote office sites. Each site requires a dedicated subnet that can support at least 120120 usable host IP addresses. What is the maximum number of subnets of this size that can be created from the allocated block?

Answer: 128 subnets

Answer

The maximum number of subnets that can be created is 128.
To support at least 120 usable host IP addresses per subnet, 7 host bits are required because 27−2=1262^7 - 2 = 126 usable hosts (26−2=622^6 - 2 = 62 is insufficient). This results in a /25/25 prefix (32−7=2532 - 7 = 25). Borrowing bits from /18/18 to /25/25 gives 25−18=725 - 18 = 7 subnet bits. The total number of subnets created is 27=1282^7 = 128.

Step-by-Step Solution

1
Calculate the required host bits (hh) for at least 120120 usable host IP addresses.
7 host bits are required (27−2=126≥1202^7 - 2 = 126 \ge 120).
6 host bits provide only 26−2=622^6 - 2 = 62 usable hosts, which is insufficient. 7 host bits provide 126 usable hosts.
2
Determine the prefix length needed for 7 host bits.
The subnet prefix length is /25/25 (32−7=2532 - 7 = 25).
An IPv4 address consists of 32 bits, so subtracting 7 host bits leaves 25 network/subnet bits.
3
Determine the number of subnet bits borrowed from the allocated block.
7 bits are borrowed (25−18=725 - 18 = 7).
The original allocation is /18/18 and the new subnets are /25/25.
4
Compute the total number of valid subnets.
The total number of subnets is 27=1282^7 = 128.
Borrowing 7 bits allows 272^7 distinct subnet combinations.

Key Concept

Subnet Sizing and Subnet Count Calculation
Estimated Time:1m 30s
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Topics

IPv4 Addressing and Subnetting

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