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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is configuring a newly created VLAN dedicated to VoIP phones. The router interface handling this subnet is configured with the network block 172.16.45.128/27172.16.45.128/27. What is the maximum number of usable host IP addresses that can be assigned to end devices within this subnet?

  1. 30Answer
  2. B
    32
  3. C
    62
  4. D
    14

Answer

The maximum number of usable host IP addresses that can be assigned is 30.
A prefix length of /27 leaves 5 host bits (32−27=532 - 27 = 5). Calculating 252^5 yields 32 total IP addresses in the subnet block. Subtracting 2 for the unassignable network and broadcast addresses results in 30 usable host IP addresses.

Step-by-Step Solution

1
Determine the number of host bits remaining from the CIDR prefix.
32−27=532 - 27 = 5 host bits
An IPv4 address consists of 32 total bits. Subtracting the prefix length gives the bits reserved for host addressing.
2
Calculate the total number of IP addresses in the subnet.
25=322^5 = 32 total IP addresses
The total number of addresses is calculated as 2 raised to the power of the host bits.
3
Subtract reserved addresses to find usable host IP addresses.
32−2=3032 - 2 = 30 usable host IP addresses
The first address in the block is reserved for the network ID and the last address is reserved for the network broadcast address.

Key Concept

IPv4 Subnet Host Calculation
Estimated Time:1m 15s
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Topics

IPv4 Addressing and Subnetting

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