200-301 Question 88
Single answerYou are configuring a new subnet for a branch office. The branch requires 28 usable IP addresses for its hosts. The subnet must be configured using the address range 192.168.10.0/24. Which subnet mask would you configure to meet this requirement?
- A
255.255.255.240
- B
255.255.255.224
- C
255.255.255.192
- D
255.255.255.248
Show answer and explanation
Correct answer: B
Explanation
To calculate the subnet mask that fits 28 usable IP addresses, we consider the formula 2^n - 2, where n is the number of host bits. The smallest subnet mask that satisfies this is 255.255.255.224, which provides 30 usable IP addresses. While 255.255.255.192 also provides enough usable addresses, it is not the most efficient option.
- A. Incorrect.
255.255.255.240 provides 16 total addresses (2^4 = 16), of which 14 are usable. This is insufficient for the 28 required usable addresses.
- B. Correct.
255.255.255.224 provides 32 total addresses (2^5 = 32), of which 30 are usable. This meets the requirement of 28 usable addresses.
- C. Incorrect.
255.255.255.192 provides 64 total addresses (2^6 = 64), of which 62 are usable. While this meets the requirement, it is not the most efficient subnet mask for 28 usable addresses.
- D. Incorrect.
255.255.255.248 provides 8 total addresses (2^3 = 8), of which 6 are usable. This is insufficient for the 28 required usable addresses.