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Difficulty: MediumIPv4 Addressing and Subnetting

An enterprise network team is allocated the IPv4 address block 10.240.16.0/2010.240.16.0/20 to provision subnets for remote branch offices. Each branch subnet must support at least 60 usable host addresses. What is the maximum number of equal-sized subnets that can be created from this address block?

Answer: 64 subnets

Answer

The maximum number of equal-sized subnets that can be created is 64.
To support 60 usable host addresses, a subnet requires 6 host bits because 26−2=622^6 - 2 = 62 usable addresses (25−2=302^5 - 2 = 30 is insufficient). A subnet with 6 host bits corresponds to a prefix length of /26 (32−6=2632 - 6 = 26). Subtracting the original block prefix (/20) from the subnet prefix (/26) yields 6 borrowed subnet bits. Calculating 262^6 gives 64 maximum equal-sized subnets.

Step-by-Step Solution

1
Calculate required host bits for 60 usable hosts
6 host bits (26−2=62≥602^6 - 2 = 62 \ge 60)
Network and broadcast addresses are reserved and cannot be assigned to hosts, requiring subtraction of 2 from total addresses.
2
Determine target prefix length
Prefix length of /26
Subtracting 6 host bits from the 32 bits of an IPv4 address results in a 26-bit network prefix.
3
Calculate borrowed subnet bits
6 subnet bits
The difference between the new prefix (/26) and the allocated prefix (/20) represents the borrowed bits (26−20=626 - 20 = 6).
4
Calculate total usable subnets
64 subnets
The formula 2s2^s, where ss is the number of borrowed bits, gives 26=642^6 = 64 available subnets.

Key Concept

IPv4 Subnetting and Host Requirement Sizing
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Topics

IPv4 Addressing and Subnetting

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