Examkin

Question

Difficulty: Very hardIPv4 Addressing and Subnetting

A network architect is allocating IPv4 subnets from the parent block 192.168.100.0/24192.168.100.0/24 for a newly constructed branch office. Following Variable Length Subnet Masking (VLSM) best practices, subnets are assigned sequentially starting from the lowest available network address in descending order of subnet size. The branch office requirements are as follows:
- Data VLAN: 6060 workstations
- VoIP VLAN: 2828 IP phones
- Management VLAN: 1212 network switch interfaces
- WAN Redundancy VLAN: 33 usable host IP addresses required (22 physical router interfaces plus 11 HSRP virtual IP)

Assuming each subnet is sized to the smallest standard CIDR block that satisfies its respective requirements, what is the starting IP address of the remaining contiguous unallocated pool in the 192.168.100.0/24192.168.100.0/24 block?

  1. A
    192.168.100.116192.168.100.116
  2. 192.168.100.120192.168.100.120Answer
  3. C
    192.168.100.128192.168.100.128
  4. D
    192.168.100.118192.168.100.118

Answer

192.168.100.120192.168.100.120 is the starting network address of the unallocated space.
The starting address of the unallocated pool is 192.168.100.120192.168.100.120. Allocating the subnets sequentially by largest size first yields: Data VLAN on 192.168.100.0/26192.168.100.0/26 (ends .63), VoIP VLAN on 192.168.100.64/27192.168.100.64/27 (ends .95), Management VLAN on 192.168.100.96/28192.168.100.96/28 (ends .111), and WAN Redundancy VLAN on 192.168.100.112/29192.168.100.112/29 (ends .119, requiring a /29/29 block to fit 33 usable IPs). The unallocated space starts at 192.168.100.120192.168.100.120.

Step-by-Step Solution

1
Sort VLAN requirements in descending order of size to apply VLSM efficiently.
Order: Data VLAN (6060 hosts) →\rightarrow VoIP VLAN (2828 hosts) →\rightarrow Management VLAN (1212 hosts) →\rightarrow WAN Redundancy VLAN (33 hosts).
Allocating largest subnets first prevents address fragmentation and ensures boundary alignment.
2
Calculate required block sizes and assign the Data VLAN.
Needs 60+2=6260 + 2 = 62 IP addresses. Next power of 22 is 6464 (/26/26). Allocated block: 192.168.100.0/26192.168.100.0/26 (192.168.100.0−192.168.100.63192.168.100.0 - 192.168.100.63).
Subnet must account for host capacity plus 11 network ID and 11 broadcast address.
3
Calculate required block size and assign the VoIP VLAN.
Needs 28+2=3028 + 2 = 30 IP addresses. Next power of 22 is 3232 (/27/27). Allocated block: 192.168.100.64/27192.168.100.64/27 (192.168.100.64−192.168.100.95192.168.100.64 - 192.168.100.95).
A /27/27 block supplies 3030 usable host IPs (32−232 - 2).
4
Calculate required block size and assign the Management VLAN.
Needs 12+2=1412 + 2 = 14 IP addresses. Next power of 22 is 1616 (/28/28). Allocated block: 192.168.100.96/28192.168.100.96/28 (192.168.100.96−192.168.100.111192.168.100.96 - 192.168.100.111).
A /28/28 block supplies 1414 usable host IPs (16−216 - 2).
5
Calculate required block size and assign the WAN Redundancy VLAN.
Needs 33 usable IPs (22 physical + 11 HSRP VIP) +2+ 2 (network + broadcast) =5= 5 total addresses. Next power of 22 is 88 (/29/29). Allocated block: 192.168.100.112/29192.168.100.112/29 (192.168.100.112−192.168.100.119192.168.100.112 - 192.168.100.119).
A /30/30 only provides 22 usable host IPs (4−24 - 2), which is insufficient for 33 usable IPs. A /29/29 prefix (88 total addresses) must be selected.
6
Determine the next available unallocated IP address.
The final allocated block ends at 192.168.100.119192.168.100.119. The remaining unallocated pool begins at 192.168.100.120192.168.100.120.
The next address immediately following 192.168.100.119192.168.100.119 is 192.168.100.120192.168.100.120.

Key Concept

VLSM Subnet Block Sizing and Address Boundary Alignment
Rate this question

Topics

IPv4 Addressing and Subnetting

More questions from this topic