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Difficulty: Very hardIPv4 Addressing and Subnetting

A network architect is designing an IPv4 subnetting plan for a new high-availability database cluster. The cluster requires IP assignments for 28 database server nodes, 1 dedicated out-of-band monitoring probe, 1 management appliance, and a redundant default gateway deployment using HSRP (which requires 2 physical interface IP addresses and 1 Virtual IP address). Furthermore, enterprise policy dictates that the provisioned subnet must accommodate at least a 25% growth headroom above the total initial IP requirements. Which CIDR prefix length represents the most efficient (smallest) IPv4 subnet that satisfies all requirements, and how many usable host IP addresses will remain unassigned after the initial deployment?

  1. /26 prefix length with 29 remaining usable host IP addressesAnswer
  2. B
    /26 prefix length with 31 remaining usable host IP addresses
  3. C
    /27 prefix length with 0 remaining usable host IP addresses
  4. D
    /25 prefix length with 93 remaining usable host IP addresses

Answer

The most efficient subnet is a /26 prefix length, which leaves 29 remaining usable host IP addresses.
The total required IP count for the deployment is 33 (28 database nodes + 1 probe + 1 appliance + 3 HSRP gateway addresses). Adding 25% headroom requires supporting at least 42 usable IP addresses. A /26 subnet yields 26−2=622^6 - 2 = 62 usable host IP addresses, which is the smallest subnet that accommodates at least 42 hosts. Subtracting the initial 33 assigned addresses leaves 29 unassigned usable host IPs.

Step-by-Step Solution

1
Calculate the total initial host IP requirement
28 (servers) + 1 (probe) + 1 (appliance) + 2 (HSRP physical interfaces) + 1 (HSRP VIP) = 33 required IP addresses.
Redundant gateway protocols such as HSRP require individual IP addresses for each physical router interface plus a shared Virtual IP.
2
Apply the 25% growth headroom expansion requirement
33 × 1.25 = 41.25, which rounds up to a minimum required capacity of 42 usable host IP addresses.
Network design specifications require provisioned subnets to accommodate growth beyond initial deployment counts.
3
Determine the smallest CIDR prefix length supporting at least 42 usable host IPs
A /27 subnet provides 2(32−27)−2=302^{(32-27)} - 2 = 30 usable host IPs (insufficient). A /26 subnet provides 2(32−26)−2=622^{(32-26)} - 2 = 62 usable host IPs (sufficient).
Subnet host capacity is calculated using the formula 2h−22^h - 2, where hh is the number of host bits remaining.
4
Calculate unassigned remaining usable host IP addresses
62 usable IPs - 33 assigned IPs = 29 remaining usable host IP addresses.
Subtracting the initial deployment count from total usable capacity yields remaining capacity for growth.

Key Concept

IPv4 Subnet Mask Calculation and Usable Host Address Estimation
Estimated Time:3m 0s
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Topics

IPv4 Addressing and Subnetting

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