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Difficulty: EasyIPv4 Addressing and Subnetting

A network administrator is configuring a subnet for a small office branch using the subnet mask 255.255.255.240255.255.255.240. What is the maximum number of usable host IP addresses available on this subnet?

  1. 14Answer
  2. B
    16
  3. C
    30
  4. D
    15

Answer

14 usable host IP addresses
A subnet mask of 255.255.255.240255.255.255.240 provides 4 host bits, resulting in 24=162^4 = 16 total IP addresses. Subtracting 2 reserved addresses (the network address and broadcast address) leaves exactly 14 usable host IP addresses.

Step-by-Step Solution

1
Determine the number of host bits from the subnet mask.
The dotted-decimal mask 255.255.255.240255.255.255.240 converted to binary has 28 network bits and 4 host bits (32−28=432 - 28 = 4).
The last octet value 240240 is 11110000211110000_2, leaving 4 trailing zeros for host addressing.
2
Calculate total IP addresses in the subnet.
24=162^4 = 16 total IP addresses.
The total address capacity of a subnet is calculated using 2h2^h, where hh is the number of host bits.
3
Calculate usable host IP addresses.
16−2=1416 - 2 = 14 usable host IP addresses.
Two IP addresses in every standard subnet are reserved: the network ID (all host bits 0) and the broadcast address (all host bits 1).

Key Concept

Calculating usable IPv4 host addresses from a subnet mask by subtracting network and broadcast addresses
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Topics

IPv4 Addressing and Subnetting

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