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Difficulty: Very hardIPv4 Addressing and Subnetting

An enterprise network administrator is implementing a Variable Length Subnet Masking (VLSM) scheme using the assigned summary prefix 172.28.128.0/18172.28.128.0/18. The design requires allocating three subnets sequentially from the beginning of this address space, prioritizing the largest requirements first:

- Subnet A (Data Center): requires support for at least 1,0001,000 usable host IP addresses.
- Subnet B (App Servers): requires support for at least 500500 usable host IP addresses.
- Subnet C (Management): requires support for at least 250250 usable host IP addresses.

After allocating these three subnets efficiently without leaving unallocated gaps between them, what is the valid usable host IPv4 address range for the next available subnet that can accommodate a dev/test segment requiring at least 120120 usable host IP addresses?

  1. 172.28.135.1172.28.135.1 to 172.28.135.126172.28.135.126Answer
  2. B
    172.28.135.0172.28.135.0 to 172.28.135.127172.28.135.127
  3. C
    172.28.136.1172.28.136.1 to 172.28.136.126172.28.136.126
  4. D
    172.28.135.1172.28.135.1 to 172.28.135.254172.28.135.254

Answer

The valid usable host address range for the next available subnet is 172.28.135.1172.28.135.1 to 172.28.135.126172.28.135.126.
Allocating subnets sequentially based on host requirements yields: Subnet A (172.28.128.0/22172.28.128.0/22, ending at 172.28.131.255172.28.131.255), Subnet B (172.28.132.0/23172.28.132.0/23, ending at 172.28.133.255172.28.133.255), and Subnet C (172.28.134.0/24172.28.134.0/24, ending at 172.28.134.255172.28.134.255). The next block starts at 172.28.135.0172.28.135.0. For 120120 usable hosts, a /25/25 subnet (126126 usable hosts) is required, giving a usable IP host range of 172.28.135.1172.28.135.1 to 172.28.135.126172.28.135.126.

Step-by-Step Solution

1
Calculate prefix length and boundary for Subnet A (1,0001,000 usable hosts)
210−2=1,022≥1,000  ⟹  /222^{10} - 2 = 1,022 \ge 1,000 \implies /22 prefix (1,0241,024 total addresses). Allocated range: 172.28.128.0172.28.128.0 to 172.28.131.255172.28.131.255.
A /22 subnet provides 1,022 usable hosts across 4 third-octet block values.
2
Calculate prefix length and boundary for Subnet B (500500 usable hosts)
29−2=510≥500  ⟹  /232^9 - 2 = 510 \ge 500 \implies /23 prefix (512512 total addresses). Starting IP: 172.28.132.0172.28.132.0. Allocated range: 172.28.132.0172.28.132.0 to 172.28.133.255172.28.133.255.
A /23 subnet provides 510 usable host addresses.
3
Calculate prefix length and boundary for Subnet C (250250 usable hosts)
28−2=254≥250  ⟹  /242^8 - 2 = 254 \ge 250 \implies /24 prefix (256256 total addresses). Starting IP: 172.28.134.0172.28.134.0. Allocated range: 172.28.134.0172.28.134.0 to 172.28.134.255172.28.134.255.
A /24 subnet provides 254 usable host addresses.
4
Determine the network address and usable host range for the dev/test subnet (120120 usable hosts)
Next available start address is 172.28.135.0172.28.135.0. 27−2=126≥120  ⟹  /252^7 - 2 = 126 \ge 120 \implies /25 prefix. Subnet range: 172.28.135.0/25172.28.135.0/25. Usable host range: 172.28.135.1172.28.135.1 to 172.28.135.126172.28.135.126.
A /25 prefix accommodates up to 126 usable hosts, excluding the network ID (.0) and broadcast address (.127).

Key Concept

Variable Length Subnet Masking (VLSM) host requirement sizing and sequential block allocation
Estimated Time:3m 0s
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Topics

IPv4 Addressing and Subnetting

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