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Difficulty: Very hardIPv4 Addressing and Subnetting

A network administrator is designing a Variable Length Subnet Mask (VLSM) scheme starting from the base address block 10.100.128.0/2110.100.128.0/21. Subnets must be allocated contiguously without unallocated gaps, prioritized from largest host requirement to smallest as follows:

- Segment 1: Requires at least 500500 usable host IP addresses
- Segment 2: Requires at least 250250 usable host IP addresses
- Segment 3: Requires at least 120120 usable host IP addresses
- Segment 4: Requires a point-to-point link accommodating exactly 22 usable host IP addresses

Which of the following statements regarding this VLSM allocation scheme are correct? (Select THREE.)

  1. The broadcast address for Segment 1 is 10.100.129.25510.100.129.255.Answer
  2. B
    Segment 2 requires a subnet mask of 255.255.254.0255.255.254.0.
  3. The valid usable host IP address range for Segment 3 is 10.100.131.110.100.131.1 through 10.100.131.12610.100.131.126.Answer
  4. D
    The network ID assigned to Segment 4 is 10.100.131.124/3010.100.131.124/30.
  5. The first usable IP address assigned to Segment 4 is 10.100.131.12910.100.131.129.Answer
  6. F
    Segment 1 provides a total of 512512 usable host IP addresses.

Answer

The correct statements are: the broadcast address for Segment 1 is 10.100.129.25510.100.129.255, the valid usable host IP address range for Segment 3 is 10.100.131.110.100.131.1 through 10.100.131.12610.100.131.126, and the first usable IP address assigned to Segment 4 is 10.100.131.12910.100.131.129.
The statement regarding Segment 1's broadcast address being 10.100.129.25510.100.129.255 is correct because a /23/23 subnet spanning 10.100.128.010.100.128.0 ends at 10.100.129.25510.100.129.255. The statement regarding Segment 3's usable range (10.100.131.110.100.131.1–10.100.131.12610.100.131.126) is correct because its /25/25 allocation spans 10.100.131.010.100.131.0 to 10.100.131.12710.100.131.127. The statement specifying Segment 4's first usable IP as 10.100.131.12910.100.131.129 is correct because Segment 4's /30/30 block begins at Network ID 10.100.131.12810.100.131.128.

Step-by-Step Solution

1
Calculate subnet requirement and bounds for Segment 1
Segment 1 needs 500 usable hosts. 29−2=510≥5002^9 - 2 = 510 \ge 500, requiring a /23/23 block (255.255.254.0255.255.254.0). Starting at 10.100.128.0/2310.100.128.0/23, the network ID is 10.100.128.010.100.128.0 and the broadcast address is 10.100.129.25510.100.129.255. Total usable hosts is 510 (512 total IPs minus 2).
VLSM allocations must satisfy host requirements using the smallest matching block (2n−22^n - 2).
2
Calculate subnet requirement and bounds for Segment 2
Segment 2 needs 250 usable hosts. 28−2=254≥2502^8 - 2 = 254 \ge 250, requiring a /24/24 block (255.255.255.0255.255.255.0). Contiguously following Segment 1, Segment 2 starts at 10.100.130.0/2410.100.130.0/24. Network ID is 10.100.130.010.100.130.0, broadcast address is 10.100.130.25510.100.130.255.
The next available IP after 10.100.129.25510.100.129.255 is 10.100.130.010.100.130.0.
3
Calculate subnet requirement and bounds for Segment 3
Segment 3 needs 120 usable hosts. 27−2=126≥1202^7 - 2 = 126 \ge 120, requiring a /25/25 block (255.255.255.128255.255.255.128). Contiguously following Segment 2, Segment 3 starts at 10.100.131.0/2510.100.131.0/25. Network ID is 10.100.131.010.100.131.0, broadcast address is 10.100.131.12710.100.131.127, usable range is 10.100.131.110.100.131.1 to 10.100.131.12610.100.131.126.
The next available IP after 10.100.130.25510.100.130.255 is 10.100.131.010.100.131.0.
4
Calculate subnet requirement and bounds for Segment 4
Segment 4 needs 2 usable hosts. 22−2=22^2 - 2 = 2, requiring a /30/30 block (255.255.255.252255.255.255.252). Contiguously following Segment 3, Segment 4 starts at 10.100.131.128/3010.100.131.128/30. Network ID is 10.100.131.12810.100.131.128, first usable IP is 10.100.131.12910.100.131.129, second usable IP is 10.100.131.13010.100.131.130, broadcast address is 10.100.131.13110.100.131.131.
The next available IP after 10.100.131.12710.100.131.127 is 10.100.131.12810.100.131.128.

Key Concept

Variable Length Subnet Masking (VLSM) and IPv4 Usable Host Calculations
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Topics

IPv4 Addressing and Subnetting

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