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Difficulty: EasyIPv4 Addressing and Subnetting

A network technician needs to assign an IPv4 subnet mask to a network segment that must accommodate 2828 usable host devices. Which CIDR notation prefix fulfills this requirement while reserving the minimal amount of unused IP addresses?

  1. A
    /28
  2. /27Answer
  3. C
    /26
  4. D
    /29

Answer

The /27 prefix is the smallest subnet mask that provides at least 28 usable host IP addresses.
A /27 prefix allocates 5 bits for host addressing (32−27=532 - 27 = 5), resulting in 25=322^5 = 32 total IP addresses. Subtracting 22 for the network and broadcast addresses leaves 3030 usable IP addresses (32−2=3032 - 2 = 30). This is the smallest standard subnet mask that accommodates 2828 hosts.

Step-by-Step Solution

1
Calculate the total required IP addresses by adding 2 reserved addresses (network ID and broadcast address) to the needed host count.
28 hosts+2 reserved=30 total addresses needed28\text{ hosts} + 2\text{ reserved} = 30\text{ total addresses needed}.
Every IPv4 subnet requires one network address and one broadcast address, which cannot be assigned to hosts.
2
Find the smallest block size (power of 2) that is greater than or equal to the total required addresses.
25=32 total addresses2^5 = 32\text{ total addresses}, requiring 55 host bits.
Subnet block sizes must be powers of two.
3
Subtract the host bits from 32 to determine the network prefix length.
32−5=/2732 - 5 = /27.
An IPv4 address contains 32 total bits.

Key Concept

IPv4 Host Capacity and Subnet Mask Selection
Estimated Time:45s
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Topics

IPv4 Addressing and Subnetting

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