N10-009 Question 118
Single answerSubnetting: Variable Length Subnet Mask (VLSM), Classless Inter-domain Routing (CIDR)A network administrator is redesigning addressing for a small campus using the 192.168.50.0/24 network. The site has three VLANs that need to be created with minimal wasted addresses: Engineering requires 60 usable host addresses, Sales requires 28 usable host addresses, and Management requires 12 usable host addresses. The administrator also wants to keep the remaining addresses available for future use. Which subnetting plan best meets these requirements using VLSM?
- A
Engineering: 192.168.50.0/26, Sales: 192.168.50.64/27, Management: 192.168.50.96/28
- B
Engineering: 192.168.50.0/25, Sales: 192.168.50.128/27, Management: 192.168.50.160/28
- C
Engineering: 192.168.50.0/26, Sales: 192.168.50.64/26, Management: 192.168.50.128/28
- D
Engineering: 192.168.50.0/27, Sales: 192.168.50.32/27, Management: 192.168.50.64/28
Show answer and explanation
Correct answer: A
Explanation
VLSM allows different subnet sizes within the same parent network so administrators can match subnet size to actual host requirements and avoid wasting address space. For this scenario, the correct subnet sizes are /26 for 60 hosts, /27 for 28 hosts, and /28 for 12 hosts. In IPv4, each subnet reserves two addresses for network and broadcast, so usable host counts are 62, 30, and 14 respectively. Starting with the largest subnet first is a standard best practice because it reduces the risk of overlap and fragmentation. This approach aligns with common IP addressing design guidance and CIDR-based subnet allocation practices defined in RFC 4632, which describes classless addressing and route aggregation principles.
- A. Correct.
Correct. A /26 provides 64 total addresses, or 62 usable, which satisfies Engineering's need for 60 hosts. A /27 provides 32 total addresses, or 30 usable, which satisfies Sales' requirement for 28 hosts. A /28 provides 16 total addresses, or 14 usable, which satisfies Management's need for 12 hosts. These subnets are contiguous, non-overlapping, and use VLSM efficiently, leaving 192.168.50.112 through 192.168.50.255 available for future allocation.
- B. Incorrect.
Incorrect. This plan works technically, but it does not meet the requirement for minimal wasted addresses. Engineering only needs 60 usable addresses, so assigning a /25 gives 126 usable addresses, wasting far more space than necessary. This is a common mistake when administrators choose a larger subnet than required instead of applying VLSM to conserve address space.
- C. Incorrect.
Incorrect. Engineering's /26 is appropriate, but Sales is assigned another /26, which provides 62 usable addresses for a requirement of 28. That wastes significant address space and does not best meet the requirement to preserve addresses for future use. Although the subnets do not overlap, this is not the most efficient VLSM design.
- D. Incorrect.
Incorrect. A /27 provides only 30 usable addresses, so it cannot support Engineering's requirement for 60 hosts. Sales would fit in a /27 and Management would fit in a /28, but the Engineering subnet is undersized. This reflects a common subnetting error of confusing total addresses with usable host capacity.