Why Router Interfaces Cannot Use Broadcast Addresses

N10-009 By islever Editorial Last updated

Short answer

B The router interface is a broadcast address.

Question

A network administrator configured a router interface as 10.0.0.95 255.255.255.240. The administrator discovers that the router is not routing packets to a web server with IP 10.0.0.81/28. Which of the following is the best explanation?
  1. The web server is in a different subnet.
  2. The router interface is a broadcast address. Correct Answer
  3. The IP address space is a class A network.
  4. The subnet is in a private address space.

Key Takeaways

Tests your ability to derive subnet ranges from CIDR notation and recognize reserved addresses, with the common trap being the assumption that similar-looking IPs automatically belong to different networks.

This CompTIA Network+ N10-009 question tests IPv4 subnet boundary calculations and host address restrictions. Community consensus confirms that assigning a broadcast address to a router interface breaks local communication because it violates fundamental IP addressing standards.

Common mistake: Option A is frequently selected when candidates skip the subnet math, incorrectly assuming the addresses fall into different ranges instead of recognizing they share the same /28 block.

Expert Analysis

Why the Answer Is Correct

A /28 prefix corresponds to a 255.255.255.240 mask, creating blocks of 16 IP addresses. For the 10.0.0.80 subnet, the usable range is 10.0.0.81 through 10.0.0.94, making 10.0.0.95 the designated broadcast address. Router interfaces must be assigned usable host addresses within a subnet, never the network or broadcast address, which explains the routing failure.

Why the Other Options Are Wrong

Option A is incorrect because both the router and server reside in the exact same /28 subnet. Option C is irrelevant since Class A/B/C boundaries are obsolete in modern CIDR routing environments. Option D is misleading because while 10.0.0.0/8 is indeed private, private addressing does not inherently block local routing or inter-subnet communication.

Community Comment Notes

Multiple high-voted comments [1][2] emphasize using the magic number method (256-240=16) to quickly map the 80-95 range. Comment [4] reinforces that exam candidates should immediately flag any interface configuration ending in the final IP of a subnet block as invalid. These explanations align perfectly with standard networking certification troubleshooting methodologies.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

Always calculate subnet ranges using the block size method before evaluating IP conflict questions. Memorize that the first IP is the network identifier and the last IP is the broadcast address, neither of which can be assigned to end devices or router interfaces.

Community Votes

B
100%

100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 4 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 4

  1. GT256 👍 4 Answered B
    answer is B 255.255.255.240 is /28, and has 16 addresses Network Address: 10.0.0.80 First usable: 10.0.0.81 Web Server Last usable: 10.0.0.94 Last IP / Broadcast IP: 10.0.0.95 Router Next network: 10.0.0.96 Maybe the network admin wanted to assign the router 10.0.0.94 (last usable IP for network 10.0.0.80/28), thankfully this shouldn't happen because companies always emphasise attention to detail on job postings Dennizje is right
  2. Dennizje 👍 3 Answered B
    It's B. The Network address is 10.0.0.80. So the printer is NOT in a different subnet. Subnet is /28. Last octet is 01011111. From the last octet, the first 4 are static. Since the last 4 bits are all occupied, it is the last possible IP address of the subnet before moving on to the next. The last possible IP address is the broadcast address. People voting A are just assuming because it's a common answer on practice tests.
  3. escomgmt 👍 2
    A, The router interface is configured with IP 10.0.0.95 and a subnet mask of 255.255.255.240, which translates to a /28 subnet (or 16 IP addresses per subnet). The address range for the subnet that includes 10.0.0.95/28 is 10.0.0.80 - 10.0.0.95. However, 10.0.0.81 is within this subnet, but 10.0.0.95 is the broadcast address for the subnet, meaning the router's configuration on 10.0.0.95 isn't appropriate for routing directly to 10.0.0.81.
  4. ec80b38 👍 2 Answered B
    B 95 is the brodcast address

Reader comments & corrections

No reader corrections yet — spotted an error or have a note? Share it below.

Log in to comment, report an error, or add a note about this question.

Why does a moved 2.4GHz WAP cause intermittent laptop issues?

S-grade

How to Reduce Network Congestion and Improve Performance?

S-grade

Why did the telephone system lose connection with a working console and module access?

S-grade

What is the transport layer transmission format for connectionless communication?

S-grade

Practice All N10-009 Questions

Access 100 questions with complete answers and detailed explanations.

View Full N10-009 Practice Test →

← Back to N10-009 Study Guide