Examkin

Question

Difficulty: HardIPv4 Addressing and Subnetting

A network engineer is implementing a Variable Length Subnet Mask (VLSM) addressing scheme for a regional facility using the summary block 10.200.16.0/2210.200.16.0/22. Subnet A must support up to 120120 usable host addresses for a departmental LAN segment. Subnet B must support a point-to-point WAN link between two routers. Subnet A is allocated the lowest numerical range within the summary block, starting at 10.200.16.010.200.16.0. If Subnet B is assigned the next immediately adjacent subnet block with the tightest prefix length that satisfies its requirement, which IPv4 address is a valid host address for Subnet B?

  1. A
    10.200.17.110.200.17.1
  2. 10.200.16.13010.200.16.130Answer
  3. C
    10.200.16.13110.200.16.131
  4. D
    172.32.16.129172.32.16.129

Answer

The IPv4 address 10.200.16.13010.200.16.130 is a valid usable host address for Subnet B.
The address 10.200.16.13010.200.16.130 is a valid usable host address on Subnet B. Subnet A requires 120120 usable host addresses, which requires 77 host bits (27−2=1262^7 - 2 = 126), resulting in a /25/25 subnet mask. Starting at 10.200.16.010.200.16.0, Subnet A occupies 10.200.16.010.200.16.0 through 10.200.16.12710.200.16.127. Subnet B starts immediately at 10.200.16.12810.200.16.128. Because Subnet B is a point-to-point link requiring 22 host IPs, a /30/30 mask (22 host bits) is assigned. The /30/30 block 10.200.16.128/3010.200.16.128/30 has usable host addresses 10.200.16.12910.200.16.129 and 10.200.16.13010.200.16.130.

Step-by-Step Solution

1
Determine the prefix length and address boundary for Subnet A
Subnet A uses a /25/25 mask, spanning 10.200.16.010.200.16.0 to 10.200.16.12710.200.16.127.
Subnet A requires 120120 usable host addresses. Using the formula 2h−2≥1202^h - 2 \ge 120, h=7h = 7 host bits are needed (27−2=1262^7 - 2 = 126). The prefix length is 32−7=/2532 - 7 = /25. Starting at 10.200.16.010.200.16.0, the subnet range runs from network ID 10.200.16.010.200.16.0 to broadcast address 10.200.16.12710.200.16.127.
2
Identify the starting network address and required mask for Subnet B
Subnet B network address is 10.200.16.12810.200.16.128 with a /30/30 prefix length.
The next immediately adjacent unallocated IP address is 10.200.16.12810.200.16.128. Subnet B needs 22 usable host addresses for a point-to-point link. Using 2h−2≥22^h - 2 \ge 2, h=2h = 2 host bits are needed (22−2=22^2 - 2 = 2). The prefix length is 32−2=/3032 - 2 = /30.
3
Calculate the valid host address range for Subnet B
Valid host addresses are 10.200.16.12910.200.16.129 and 10.200.16.13010.200.16.130.
For the network 10.200.16.128/3010.200.16.128/30, 10.200.16.12810.200.16.128 is the network address, 10.200.16.12910.200.16.129 is the first usable host, 10.200.16.13010.200.16.130 is the last usable host, and 10.200.16.13110.200.16.131 is the broadcast address.

Key Concept

VLSM block allocation and usable host range calculation for sequential subnets
Estimated Time:2m 0s
Rate this question

Topics

IPv4 Addressing and Subnetting

More questions from this topic