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220-1201 Practice Question: A small office uses a single router with a…
A small office uses a single router with a built-in DHCP server. The office has 15 devices that need IP addresses. The router's default subnet mask is 255.255.255.0. How many usable IP addresses are available for devices on this subnet?
⚠ Common exam trap
The 220-1201 exam often tests the distinction between total addresses and usable addresses, trapping candidates who forget to subtract the network and broadcast addresses, leading them to choose 256 instead of 254.
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
✓
254
The subnet mask 255.255.255.0 corresponds to a /24 prefix, which provides 256 total addresses (2^8). However, the first address (network address) and the last address (broadcast address) are reserved, leaving 254 usable IP addresses for devices. This is a fundamental IPv4 subnetting rule defined in RFC 791.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
254
Why this is correct
A /24 network, indicated by a subnet mask of 255.255.255.0, allocates 8 bits for host addresses. This provides 2^8, or 256, total possible addresses within that subnet. However, two addresses are always reserved: the network address (where all host bits are zero) and the broadcast address (where all host bits are one). Subtracting these two reserved addresses leaves 254 usable IP addresses for assigning to individual hosts and devices on the network.
- ✗
255
Why it's wrong here
The value 255, when appearing as the last octet in an IPv4 address within a /24 subnet (e.g., 192.168.1.255), specifically designates the broadcast address for that network segment. This address is reserved for sending data to all devices simultaneously on the local network, rather than to a single host. Consequently, it cannot be assigned to an individual host or router interface as a usable IP address, making it an incorrect count for assignable hosts.
- ✗
256
Why it's wrong here
While a /24 subnet mathematically encompasses 256 distinct IP addresses, this number represents the total address space, not the usable count for hosts. The 256 addresses are derived from 2 to the power of 8 (2^8), reflecting the 8 bits available for the host portion of the IP address. However, within this total, the very first address (all host bits zero) is reserved as the network identifier, and the very last address (all host bits one) is reserved for broadcast purposes, rendering them unusable for host assignment.
- ✗
15
Why it's wrong here
The number 15 refers to the quantity of devices currently present in the small office, as mentioned in the question stem, not the maximum capacity of the subnet. The question specifically asks for the maximum number of usable IP addresses that the router's subnet provides, which is a function of the subnet mask, not the current device count. Therefore, 15 is irrelevant to the calculation of available IP addresses within the network's defined address range.
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1201 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 220-1201 exam.