Cisco CCNA (200-301)Network FundamentalsHard
An engineer must subnet the 192.168.20.0/24 network using VLSM to support a LAN segment requiring 100 hosts. What is the correct subnet mask to use for this segment to minimize wasted addresses?
- A255.255.255.0
- B255.255.255.128
- C255.255.255.224
- D255.255.255.192
Show answer & explanationAnswer & explanation
Correct answer: B. 255.255.255.128
A /25 mask (255.255.255.128) provides 2^7-2 = 126 usable host addresses, the smallest block that satisfies the 100-host requirement. A /26 only offers 62 usable hosts, which is insufficient.
Why the other options are wrong
- A. /24 gives 254 usable hosts, far more than necessary, wasting address space.
- C. /27 gives only 30 usable hosts, nowhere near enough.
- D. /26 gives only 62 usable hosts, which is not enough for 100 hosts.
VLSM Subnet Sizing
Variable Length Subnet Masking allows different subnet sizes within the same major network, sized to match each segment's host requirement to minimize address waste.
- Formula: 2^n - 2 >= required hosts, solve for smallest n
- /25 = 126 usable hosts, fits 100-host requirement
- VLSM assigns the largest subnets first when dividing address space
Memory trick: Fit the subnet like a tailored suit—not too big, not too small.