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Difficulty: Very hardIPv4 Addressing and Subnetting

A network architect is designing a Variable Length Subnet Mask (VLSM) IP addressing scheme starting from the base network block 172.16.64.0/19172.16.64.0/19. The subnets must be allocated contiguously without wasting address space in the following order:

1. Subnet A (Server Farm): Requires support for at least 1,0001,000 usable host IP addresses.
2. Subnet B (Corporate Clients): Requires support for at least 450450 usable host IP addresses.
3. Subnet C (VoIP Infrastructure): Requires support for at least 100100 usable host IP addresses.

Assuming each subnet is assigned the most efficient mask possible to satisfy host requirements, what is the broadcast IP address of Subnet C?

  1. 172.16.70.127Answer
  2. B
    172.16.70.255
  3. C
    172.16.74.127
  4. D
    172.16.70.128

Answer

172.16.70.127
The option specifying 172.16.70.127 accurately calculates the minimum host bit requirements for each sequential VLSM allocation (10 bits for Subnet A, 9 bits for Subnet B, and 7 bits for Subnet C), placing Subnet C at 172.16.70.0/25 with a broadcast address of 172.16.70.127.

Step-by-Step Solution

1
Calculate mask and range for Subnet A (1,000 hosts)
Network: 172.16.64.0/22, Range: 172.16.64.0 to 172.16.67.255
For 1,000 hosts, formula 2^h - 2 >= 1,000 requires h = 10 bits (2^10 - 2 = 1,022 usable hosts). Prefix length is 32 - 10 = /22 (block size of 4 in the 3rd octet).
2
Calculate mask and range for Subnet B (450 hosts)
Network: 172.16.68.0/23, Range: 172.16.68.0 to 172.16.69.255
Subnet B begins immediately after Subnet A at 172.16.68.0. For 450 hosts, 2^h - 2 >= 450 requires h = 9 bits (2^9 - 2 = 510 usable hosts). Prefix length is 32 - 9 = /23 (block size of 2 in the 3rd octet).
3
Calculate mask, range, and broadcast for Subnet C (100 hosts)
Network: 172.16.70.0/25, Broadcast: 172.16.70.127
Subnet C begins immediately after Subnet B at 172.16.70.0. For 100 hosts, 2^h - 2 >= 100 requires h = 7 bits (2^7 - 2 = 126 usable hosts). Prefix length is 32 - 7 = /25 (block size of 128 in the 4th octet). The range is 172.16.70.0 to 172.16.70.127, making 172.16.70.127 the broadcast address.

Key Concept

Variable Length Subnet Masking (VLSM) Host Capacity Allocation
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Topics

IPv4 Addressing and Subnetting

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