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Difficulty: MediumIPv4 Addressing and Subnetting

A system administrator is designing an isolated VLAN for a database cluster. The cluster requires 2727 usable IPv4 addresses for database nodes, 11 address for the default gateway, and 22 addresses for virtual IP load balancers. Which of the following represents the smallest CIDR prefix mask that provides sufficient usable IPv4 addresses for all required devices?

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

Answer

The smallest CIDR prefix mask that satisfies the requirement is /27/27.
The scenario requires 3030 usable IP addresses (27+1+2=3027 + 1 + 2 = 30). A /27/27 subnet has 55 host bits (32−27=532 - 27 = 5), providing 25=322^5 = 32 total IP addresses. Subtracting 22 for the network address and broadcast address leaves exactly 3030 usable host addresses (32−2=3032 - 2 = 30), making /27/27 the smallest prefix mask that satisfies the requirement.

Step-by-Step Solution

1
Calculate total usable IPv4 addresses required.
27 nodes+1 gateway+2 load balancers=30 usable host IP addresses27 \text{ nodes} + 1 \text{ gateway} + 2 \text{ load balancers} = 30 \text{ usable host IP addresses}.
All active host interfaces on the network segment require individual usable IP addresses.
2
Determine the required host bits (hh) using the formula for usable hosts.
2h−2≥30  ⟹  2h≥32  ⟹  h=52^h - 2 \ge 30 \implies 2^h \ge 32 \implies h = 5 host bits.
Two IP addresses in every standard subnet are reserved for the network ID and broadcast address.
3
Calculate the CIDR prefix length.
32−5=2732 - 5 = 27, resulting in a /27/27 subnet mask.
An IPv4 address consists of 32 bits; subtracting 5 host bits leaves 27 prefix bits for the network.

Key Concept

Calculating required subnet size and usable host counts using CIDR notation
Estimated Time:1m 30s
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Topics

IPv4 Addressing and Subnetting

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