100-150 Question 56
Single answerYou are tasked with configuring a subnet for a small office network that requires a maximum of 30 usable IP addresses. The network administrator has assigned you the IP block 192.168.1.0/24. Which subnet mask would you use to meet the requirements while minimizing unused IP addresses?
- 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 determine the appropriate subnet mask, you calculate the number of IP addresses needed to meet the requirement of 30 usable addresses. Subnetting reduces the size of the available address space by increasing the number of bits in the subnet mask. A /27 mask (255.255.255.224) provides exactly 30 usable addresses, which is the most efficient choice. Choosing a /26 would exceed the requirement and waste addresses, while smaller subnet masks like /28 or /29 would not meet the requirement.
- A. Incorrect.
This subnet mask provides 64 total IP addresses (2^6), with 62 usable addresses after subtracting two for the network and broadcast addresses. This exceeds the requirement of 30 usable IP addresses, resulting in unnecessary waste.
- B. Correct.
This subnet mask provides 32 total IP addresses (2^5), with 30 usable addresses after subtracting two for the network and broadcast addresses. This perfectly meets the requirement without wasting additional IP addresses.
- C. Incorrect.
This subnet mask provides 16 total IP addresses (2^4), with 14 usable addresses after subtracting two for the network and broadcast addresses. This does not meet the requirement of 30 usable IP addresses.
- D. Incorrect.
This subnet mask provides 8 total IP addresses (2^3), with 6 usable addresses after subtracting two for the network and broadcast addresses. This does not meet the requirement of 30 usable IP addresses.