200-301 Question 275
Single answerA network administrator is tasked with subnetting a Class C network (192.168.10.0/24) into smaller subnets to support 6 separate VLANs. Each VLAN requires at least 20 usable IP addresses. Which subnet mask should the administrator use to meet the requirements?
- A
255.255.255.192 (/26)
- B
255.255.255.224 (/27)
- C
255.255.255.240 (/28)
- D
255.255.255.248 (/29)
Show answer and explanation
Correct answer: B
Explanation
To subnet the network into 6 VLANs with at least 20 usable IP addresses per VLAN, the administrator must select a subnet mask that provides sufficient usable addresses and allows for the creation of at least 6 subnets. The /27 subnet mask (255.255.255.224) satisfies both conditions: it provides 30 usable IP addresses per subnet and allows for up to 8 subnets in the 192.168.10.0/24 network.
- A. Incorrect.
A /26 subnet mask (255.255.255.192) provides 62 usable IP addresses (2^6 - 2). While this exceeds the requirement of 20 usable IP addresses per subnet, it does not allow for 6 subnets because only 4 subnets are possible with a /26 mask.
- B. Correct.
A /27 subnet mask (255.255.255.224) provides 30 usable IP addresses (2^5 - 2) per subnet, which meets the requirement of at least 20 usable IP addresses. Additionally, a /27 subnet mask allows for 8 subnets, which is sufficient to support 6 VLANs.
- C. Incorrect.
A /28 subnet mask (255.255.255.240) provides 14 usable IP addresses (2^4 - 2), which does not meet the requirement of at least 20 usable IP addresses per VLAN.
- D. Incorrect.
A /29 subnet mask (255.255.255.248) provides only 6 usable IP addresses (2^3 - 2), which is far below the requirement of at least 20 usable IP addresses per VLAN.