DevNet Associate (DEVASC) v1.0Network FundamentalsHard
A network engineer is configuring a new subnet for a small branch office that will host a maximum of 25 devices, including computers, printers, and VoIP phones. To minimize wasted IP addresses while accommodating future growth slightly, what is the smallest subnet mask that should be used?
- A255.255.255.240
- B255.255.255.192
- C255.255.255.224
- D255.255.255.248
Show answer & explanationAnswer & explanation
Correct answer: C. 255.255.255.224
We need to find a subnet that can accommodate at least 25 devices. The formula for usable hosts is 2^n - 2, where n is the number of host bits. For 25 devices, we need n such that 2^n - 2 >= 25. If n=4, 2^4 - 2 = 14 (too small). If n=5, 2^5 - 2 = 30 (sufficient). A subnet with 5 host bits means 32 - 5 = 27 network bits. This corresponds to a /27 CIDR mask. In dotted-decimal, /27 is 255.255.255.224.
Why the other options are wrong
- A. 255.255.255.240 is a /28, providing 14 usable hosts (2^4 - 2 = 14), which is too small for 25 devices.
- B. 255.255.255.192 is a /26, providing 62 usable hosts (2^6 - 2 = 62), which is much larger than needed and wastes more addresses.
- D. 255.255.255.248 is a /29, providing 6 usable hosts (2^3 - 2 = 6), which is too small.
Subnet Mask Calculation (VLSM)
The process of determining the appropriate subnet mask to create subnets of a specific size, minimizing IP address waste.
- Number of usable hosts = 2^n - 2, where n is the number of host bits.
- A smaller 'n' (fewer host bits) means a larger subnet mask and fewer hosts.
- A /27 mask provides 30 usable hosts (2^5 - 2).
Memory trick: Host Bits: How Many To Serve?