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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is provisioning a dedicated IPv4 subnet for a web server cluster that requires static IP address assignments for 2828 physical web servers, 22 firewall gateway interfaces, and 11 floating virtual IP (VIP) address. Assuming the administrator assigns the smallest standard CIDR subnet block that accommodates all required endpoints, how many additional usable host IP addresses will remain available for future expansion?

  1. 3131Answer
  2. B
    3333
  3. C
    11
  4. D
    9595

Answer

The smallest CIDR subnet that accommodates the requirement leaves 31 additional usable host IP addresses available.
The scenario requires 3131 static IP assignments (2828 servers + 22 gateways + 11 VIP). A /27/27 subnet block contains 3232 total IP addresses, but only 25−2=302^5 - 2 = 30 are usable, which falls short by one address. Consequently, the administrator must step up to a /26/26 subnet block (6464 total addresses), which yields 26−2=622^6 - 2 = 62 usable host IPs. Subtracting the 3131 assigned addresses from the 6262 usable addresses leaves exactly 3131 usable host IP addresses available for future expansion.

Step-by-Step Solution

1
Calculate the total number of required host IP endpoints.
28 servers+2 gateway interfaces+1 VIP=31 required IP addresses28\text{ servers} + 2\text{ gateway interfaces} + 1\text{ VIP} = 31\text{ required IP addresses}.
Every active host interface, physical gateway, and virtual redundancy IP requires a distinct usable IP address.
2
Determine the smallest subnet prefix that supports at least 31 usable IP addresses.
A /27/27 prefix yields 25−2=302^5 - 2 = 30 usable addresses (insufficient). A /26/26 prefix yields 26−2=622^6 - 2 = 62 usable addresses (sufficient).
Usable host capacity formula is 2h−22^h - 2, where hh is the number of host bits remaining (32−prefix32 - \text{prefix}).
3
Calculate the remaining available usable IP addresses in the /26/26 subnet.
62 usable addresses−31 assigned addresses=31 available addresses62\text{ usable addresses} - 31\text{ assigned addresses} = 31\text{ available addresses}.
Subtracting the required IP addresses from the maximum usable host capacity gives the remaining expansion capacity.

Key Concept

Usable IPv4 Host Calculation & Subnet Sizing
Estimated Time:1m 30s
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Topics

IPv4 Addressing and Subnetting

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