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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is designing an IPv4 addressing scheme for a security monitoring VLAN that must support 3131 dedicated IP cameras, each requiring a static usable IPv4 address. Which CIDR prefix length represents the smallest subnet block that accommodates all required cameras while conserving address space?

  1. /26/26Answer
  2. B
    /27/27
  3. C
    /28/28
  4. D
    /25/25

Answer

The prefix /26/26 is the correct choice because it provides 6262 usable host IP addresses, satisfying the requirement for 3131 cameras with minimal address waste.
The prefix /26/26 allocates 66 host bits, providing 26−2=622^6 - 2 = 62 usable host IP addresses. This is the smallest standard IPv4 subnet that accommodates 3131 devices without exhausting available addresses.

Step-by-Step Solution

1
Determine the required number of usable host IP addresses
The network must support at least 3131 usable host IP addresses.
Each IP camera requires its own individual static usable IPv4 address.
2
Calculate usable host capacity using the formula 2h−2≥312^h - 2 \ge 31
For h=5h = 5 host bits, 25−2=302^5 - 2 = 30 usable hosts (insufficient). For h=6h = 6 host bits, 26−2=622^6 - 2 = 62 usable hosts (sufficient).
Two IP addresses in every IPv4 subnet are reserved for the network ID and broadcast address and cannot be assigned to host interfaces.
3
Convert host bits to CIDR prefix length
Prefix length =32−6=/26= 32 - 6 = /26.
An IPv4 address consists of 3232 total bits, so subtracting the 66 host bits leaves 2626 prefix bits.

Key Concept

Subnet Usable Host Calculation
Estimated Time:1m 30s
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Topics

IPv4 Addressing and Subnetting

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