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Zorluk: ZorIPv4 Addressing and Subnetting

A network administrator is designing a VLSM addressing scheme for a branch office using the parent IPv4 block 172.31.96.0/21172.31.96.0/21. The subnets must be allocated contiguously starting from the lowest available address in the block, ordered by host requirements from largest to smallest:

- VLAN 10 requires at least 130 usable host IP addresses
- VLAN 20 requires at least 60 usable host IP addresses
- VLAN 30 requires at least 28 usable host IP addresses

What is the last usable IPv4 host address allocated to VLAN 30?

  1. 172.31.97.94172.31.97.94Cevap
  2. B
    172.31.97.95172.31.97.95
  3. C
    172.32.97.94172.32.97.94
  4. D
    172.31.97.62172.31.97.62

Cevap

The last usable IPv4 host address allocated to VLAN 30 is 172.31.97.94172.31.97.94.
Allocating subnets using VLSM in descending order gives VLAN 10 the block 172.31.96.0/24172.31.96.0/24 (range 172.31.96.0172.31.96.0–172.31.96.255172.31.96.255) and VLAN 20 the block 172.31.97.0/26172.31.97.0/26 (range 172.31.97.0172.31.97.0–172.31.97.63172.31.97.63). VLAN 30 receives the next available block 172.31.97.64/27172.31.97.64/27. The broadcast address of this block is 172.31.97.95172.31.97.95, making the last usable host address 172.31.97.94172.31.97.94.

Adım Adım Çözüm

1
Calculate the prefix size and subnet range for VLAN 10 (130 hosts).
To support 130 hosts, 28−2=2542^8 - 2 = 254 usable hosts are required, requiring 8 host bits (/24 mask). Starting at 172.31.96.0172.31.96.0, the subnet is 172.31.96.0/24172.31.96.0/24, covering 172.31.96.0172.31.96.0 through 172.31.96.255172.31.96.255.
VLSM allocations must satisfy 2h−2≥required hosts2^h - 2 \ge \text{required hosts} and align on bit boundaries.
2
Calculate the prefix size and subnet range for VLAN 20 (60 hosts).
To support 60 hosts, 26−2=622^6 - 2 = 62 usable hosts are required, requiring 6 host bits (/26 mask). The next available address is 172.31.97.0172.31.97.0. The subnet is 172.31.97.0/26172.31.97.0/26, covering 172.31.97.0172.31.97.0 through 172.31.97.63172.31.97.63.
Subnets must be assigned sequentially starting immediately after the preceding subnet's broadcast address.
3
Calculate the prefix size and subnet range for VLAN 30 (28 hosts).
To support 28 hosts, 25−2=302^5 - 2 = 30 usable hosts are required, requiring 5 host bits (/27 mask). The next available address is 172.31.97.64172.31.97.64. The subnet is 172.31.97.64/27172.31.97.64/27, covering network ID 172.31.97.64172.31.97.64 to broadcast 172.31.97.95172.31.97.95.
A /27 block provides 32 total addresses (252^5), yielding a broadcast address of 172.31.97.64+31=172.31.97.95172.31.97.64 + 31 = 172.31.97.95.
4
Determine the last usable host IP address for VLAN 30.
The broadcast address is 172.31.97.95172.31.97.95, so the last usable host address is 172.31.97.95−1=172.31.97.94172.31.97.95 - 1 = 172.31.97.94.
The usable host range excludes the network ID and the broadcast address.

Anahtar Kavram

Variable Length Subnet Masking (VLSM) and Usable Host Range Derivation
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Konular

IPv4 Addressing and Subnetting

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