Examkin

Question

Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is assigned the IPv4 address block 172.28.40.0/23172.28.40.0/23 to provision subnets for new departmental VLANs. Each VLAN requires a minimum of 31 usable host IP addresses. What is the maximum number of subnets meeting this requirement that can be created from the assigned block?

  1. 8Answer
  2. B
    16
  3. C
    4
  4. D
    2

Answer

The maximum number of valid subnets meeting the requirement is 8.
To support a minimum of 31 usable host IPs, the usable host capacity formula 2h−2≥312^h - 2 \ge 31 must be evaluated. With 5 host bits, 25−2=302^5 - 2 = 30 addresses are available, which is insufficient. Therefore, 6 host bits are required, providing 26−2=622^6 - 2 = 62 usable host IPs. Subtracting 6 host bits from 32 gives a prefix length of /26/26. Dividing the parent /23/23 block into /26/26 subnets borrows 26−23=326 - 23 = 3 bits, yielding 23=82^3 = 8 maximum subnets.

Step-by-Step Solution

1
Determine the required host bits for each subnet
6 host bits are needed (26−2=622^6 - 2 = 62 usable addresses). 5 host bits only yield 30 usable addresses (25−2=302^5 - 2 = 30), which fails the 31 host requirement.
Every IPv4 subnet reserves the first address for network identification and the last address for directed broadcast, requiring 2 addresses to be subtracted from the total host capacity.
2
Calculate the subnet prefix length
Subnet mask prefix length is /26/26 (32−6=2632 - 6 = 26).
An IPv4 address contains 32 total bits; subtracting 6 host bits leaves 26 network bits.
3
Calculate the total number of subnets from the parent block
226−23=23=82^{26 - 23} = 2^3 = 8 subnets.
Subdividing a /23/23 block into /26/26 subnets borrows 3 additional subnet bits (26−23=326 - 23 = 3).

Key Concept

Subnet Host Capacity Formula and Prefix Allocation
Estimated Time:1m 30s
Rate this question

Topics

IPv4 Addressing and Subnetting

More questions from this topic