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220-1201 Practice Question: A technician is configuring a small branch office…
A technician is configuring a small branch office network with 30 devices. The network uses a single router with DHCP enabled. To conserve IP addresses, the technician decides to use a subnet that provides exactly enough usable addresses for the devices, with minimal waste. Which subnet mask should the technician choose?
⚠ Common exam trap
CompTIA A+ subnetting questions often require you to remember that the router/gateway interface consumes a usable host address on the subnet. Candidates frequently pick the mask that yields exactly the number of usable addresses equal to the device count (e.g., /27 for 30 devices) and forget to reserve an address for the gateway, or they ignore the 'minimal waste' constraint and choose an unnecessarily large subnet.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
255.255.255.192
The branch office has 30 devices, but the router interface on that subnet also requires an IP address, so at least 31 usable host addresses are needed. A /27 subnet (255.255.255.224) provides only 30 usable addresses (2^5 - 2 = 30), which is not enough once the gateway is counted. A /26 subnet (255.255.255.192) provides 62 usable addresses (2^6 - 2 = 62), which accommodates the 30 devices plus the router with the least waste among the viable options. Therefore, 255.255.255.192 is the correct choice.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
255.255.255.0
Why it's wrong here
Incorrect. A /24 provides 254 usable addresses, which is far more than needed and wastes addresses.
- ✗
255.255.255.224
Why it's wrong here
A /27 mask yields 30 usable host addresses after reserving network and broadcast, matching the 30 devices exactly with no waste. Larger masks such as /26 would waste addresses, so 255.255.255.224 satisfies the minimal-waste constraint.
- ✗
255.255.255.240
Why it's wrong here
A /28 supplies only 14 usable hosts, fewer than the 30 devices require, so DHCP would exhaust the pool. It is tempting because /28 conserves addresses aggressively, and it would be correct for a segment of up to 14 devices.
- ✓
255.255.255.192
Why this is correct
A /26 provides 62 usable hosts, still leaving 32 addresses unused beyond the 30 devices, so it does not achieve minimal waste. It is tempting because /26 is a common choice for small subnets, and it would be correct if roughly 60 devices needed addresses.
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Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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