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Difficulty: HardIPv4 Addressing and Subnetting

A network engineer is designing a Variable Length Subnet Mask (VLSM) IP addressing scheme for a newly provisioned branch office using the summary block 192.168.10.0/24192.168.10.0/24. Subnets must be allocated sequentially without wasting address space, starting from the lowest available IP address in descending order of host requirements:

- Subnet A (Staff): Requires 5555 usable host IPv4 addresses
- Subnet B (Sales): Requires 2828 usable host IPv4 addresses
- Subnet C (VoIP): Requires 1212 usable host IPv4 addresses
- Subnet D (WAN Link): Requires 22 usable host IPv4 addresses

What is the broadcast address of Subnet C?

  1. A
    192.168.10.110
  2. 192.168.10.111Answer
  3. C
    192.168.10.127
  4. D
    172.32.10.111

Answer

The broadcast address of Subnet C is 192.168.10.111.
Subnet C requires 12 host addresses. Satisfying 2n−2≥122^n - 2 \ge 12 requires n=4n=4 host bits (1414 usable IPs), which yields a /28/28 prefix with a block size of 16. Allocating Subnet A (5555 hosts →/26\rightarrow /26, range .0–.63) and Subnet B (2828 hosts →/27\rightarrow /27, range .64–.95) places Subnet C at network address 192.168.10.96. Adding the block size of 16 gives the subnet boundary 192.168.10.96 to 192.168.10.111, making 192.168.10.111 the broadcast address.

Step-by-Step Solution

1
Determine the prefix and range for Subnet A (55 hosts)
Subnet A requires a /26 prefix (block size 64). Subnet A spans 192.168.10.0 to 192.168.10.63.
Formula 2n−2≥552^n - 2 \ge 55 yields n=6n=6 host bits (6262 usable IPs). The prefix is /26/26 (32−632-6).
2
Determine the prefix and range for Subnet B (28 hosts)
Subnet B starts at 192.168.10.64 with a /27 prefix (block size 32). Subnet B spans 192.168.10.64 to 192.168.10.95.
Formula 2n−2≥282^n - 2 \ge 28 yields n=5n=5 host bits (3030 usable IPs). The prefix is /27/27 (32−532-5).
3
Calculate the network address, block size, and broadcast address for Subnet C (12 hosts)
Subnet C starts at 192.168.10.96 with a /28 prefix (block size 16). The network range is 192.168.10.96/28, giving a broadcast address of 192.168.10.111.
Formula 2n−2≥122^n - 2 \ge 12 yields n=4n=4 host bits (1414 usable IPs). The prefix is /28/28 (32−432-4). Adding the block size of 16 to the network start 192.168.10.96 yields a range of 192.168.10.96 to 192.168.10.111.

Key Concept

VLSM block allocation order and subnet broadcast address determination
Estimated Time:2m 0s
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Topics

IPv4 Addressing and Subnetting

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