N10-009 Question 119
Single answerSubnetting: Variable Length Subnet Mask (VLSM), Classless Inter-domain Routing (CIDR)A network administrator is assigned the CIDR block 10.20.40.0/24 for a new branch office. The office has three VLANs that must be addressed using VLSM: Engineering needs 90 usable host addresses, Sales needs 28 usable host addresses, and a Management network needs 12 usable host addresses. The administrator wants to conserve address space and leave the remaining addresses available for future use. Which of the following subnet allocations best meets these requirements without wasting addresses or creating overlap?
- A
Engineering: 10.20.40.0/25, Sales: 10.20.40.128/27, Management: 10.20.40.160/28
- B
Engineering: 10.20.40.0/26, Sales: 10.20.40.64/27, Management: 10.20.40.96/28
- C
Engineering: 10.20.40.0/25, Sales: 10.20.40.64/27, Management: 10.20.40.96/28
- D
Engineering: 10.20.40.0/25, Sales: 10.20.40.128/28, Management: 10.20.40.144/28
Show answer and explanation
Correct answer: A
Explanation
This question tests practical use of CIDR and VLSM within a single assigned network. With VLSM, the administrator should allocate the largest subnet first, then continue with smaller subnets from the remaining address space. For 90 hosts, the smallest suitable subnet is /25 because /26 supports only 62 usable hosts. For 28 hosts, the smallest suitable subnet is /27 with 30 usable hosts. For 12 hosts, the smallest suitable subnet is /28 with 14 usable hosts. Proper subnet alignment is also critical: subnets must start on valid network boundaries for their prefix length and must not overlap. These principles are consistent with standard IPv4 subnetting practices described in RFC 4632 for CIDR and commonly applied in enterprise network design.
- A. Correct.
Correct. A /25 provides 128 total addresses, or 126 usable, which satisfies Engineering's requirement for 90 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 requirement for 12 hosts. These subnets are properly aligned on valid CIDR boundaries and do not overlap: 10.20.40.0-127, 128-159, and 160-175. This uses VLSM efficiently while leaving the rest of the /24 available for future growth.
- B. Incorrect.
Incorrect. A /26 provides only 64 total addresses, or 62 usable, which is not enough for Engineering's 90-host requirement. This is a common error when administrators underestimate usable host counts by looking only at the prefix length without calculating host capacity.
- C. Incorrect.
Incorrect. Although the subnet sizes themselves could support the required host counts, the allocations overlap. Engineering's 10.20.40.0/25 uses the range 10.20.40.0-127, so assigning Sales to 10.20.40.64/27 and Management to 10.20.40.96/28 places those networks inside the Engineering subnet. Overlapping subnets on the same routed network cause addressing and routing problems.
- D. Incorrect.
Incorrect. Engineering's /25 is valid, and Management's /28 could support 12 hosts, but Sales is assigned a /28, which provides only 14 usable addresses. Sales requires 28 usable addresses, so it needs at least a /27. This distractor reflects a common mistake of choosing the next smaller subnet without verifying usable host counts.