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Zorluk: ZorIPv4 Addressing and Subnetting

A network administrator is designing a Variable Length Subnet Mask (VLSM) scheme starting from the 172.16.128.0/18172.16.128.0/18 network block. Department A requires a subnet that supports at least 500500 usable host addresses. If the administrator allocates the first available subnet of the minimum required size to Department A, which TWO statements regarding this allocation are true?

  1. The subnet mask required for Department A is 255.255.254.0255.255.254.0.Cevap
  2. The broadcast address for Department A's allocated subnet is 172.16.129.255172.16.129.255.Cevap
  3. C
    The allocated subnet provides a maximum of 512512 usable host addresses for Department A.
  4. D
    The allocated subnet address block falls outside the RFC 1918 private IPv4 address space.

Cevap

The correct statements are that the subnet mask required for Department A is 255.255.254.0255.255.254.0 and the broadcast address for Department A's allocated subnet is 172.16.129.255172.16.129.255.
To support 500 usable host addresses, 9 host bits are necessary because 29−2=5102^9 - 2 = 510 usable addresses. This yields a /2323 prefix, which translates to a subnet mask of 255.255.254.0255.255.254.0. Allocating the first available /2323 block from 172.16.128.0/18172.16.128.0/18 results in the network address 172.16.128.0172.16.128.0 and broadcast address 172.16.129.255172.16.129.255.

Adım Adım Çözüm

1
Determine the required host bits and prefix length for Department A.
Department A requires ≥500\ge 500 usable hosts. Using the formula 2h−2≥5002^h - 2 \ge 500, h=9h = 9 host bits (29−2=5102^9 - 2 = 510 usable host addresses). The prefix length is 32−9=/2332 - 9 = /23.
Host bits must satisfy the required host capacity while preserving network bits.
2
Convert prefix length /2323 to dotted-decimal subnet mask format.
A /2323 prefix corresponds to mask 255.255.254.0255.255.254.0.
The first 23 bits are set to 1 (11111111.11111111.11111110.0000000011111111.11111111.11111110.00000000).
3
Calculate the subnet boundaries for the first /2323 block within 172.16.128.0/18172.16.128.0/18.
Network address: 172.16.128.0/23172.16.128.0/23. Usable range: 172.16.128.1172.16.128.1 – 172.16.129.254172.16.129.254. Broadcast address: 172.16.129.255172.16.129.255.
The block size in the 3rd octet is 256−254=2256 - 254 = 2, covering 3rd octet values 128 and 129.
4
Verify RFC 1918 private address classification.
The range 172.16.128.0172.16.128.0–172.16.129.255172.16.129.255 is inside the 172.16.0.0/12172.16.0.0/12 private block.
RFC 1918 designates 172.16.0.0172.16.0.0 to 172.31.255.255172.31.255.255 as private IPv4 space.

Anahtar Kavram

VLSM Subnet Sizing and Boundary Calculation
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IPv4 Addressing and Subnetting

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