Class B /16 to /17 Subnetting: Which Concept Applies?

N10-009 By islever Editorial Last updated

Given a scenario, use appropriate IP v4 network addressing.

Short answer

C VLSM, because changing the Class B mask from 255.255.0.0 (/16) to 255.255.128.0 (/17) borrows one host bit and creates two subnets.

Question

A network administrator needs to split the company network into two subnets. The company has a Class В address, and the current mask in use is 255.255.0.0. The administrator realizes that a mask of 255.255.128.0 will accomplish the division. Which of the following concepts describes this change?
  1. APIРА
  2. RFC1918
  3. VLSM Correct Answer
  4. Class С

Key Takeaways

The question tests whether you recognize a non-default subnet mask as VLSM rather than confusing the smaller subnet with Class C, RFC 1918, or APIPA.

Changing a Class B network from 255.255.0.0 to 255.255.128.0 creates two /17 subnets by borrowing one host bit, and the concept tested is VLSM (C).

Common mistake: Choosing D, Class C, because the new /17 mask reduces the host space; however, Class C is determined by the 192–223 first octet and a /24 default, not by re-masking a Class B address.

Expert Analysis

Why the mask change matters The stem starts with a Class B address and a default mask of 255.255.0.0, which is /16. Replacing it with 255.255.128.0 adds one network bit and makes the mask /17. That borrowed bit divides the original /16 block into two equal /17 blocks. The address itself remains a Class B address; the change is in how many bits are treated as network bits, not in the address class.

The attractive wrong turn: Class C Option D sounds plausible because the new mask is “smaller” than the old one and leaves fewer hosts per subnet, which feels like moving to a smaller address class. That is the trap. Class C is defined by the first octet, 192–223, and its classful mask is /24, not /17. The company still has a Class B prefix; it is simply subnetting that prefix. A /17 mask on a Class B address does not turn it into Class C.

Why VLSM is the intended concept Option C, VLSM, is the best fit among the choices. A comment on this question explains it as borrowing one bit from the host portion to split the Class B /16 into two smaller subnets, which matches the arithmetic: 2^1 = 2 subnets. In CompTIA’s scenario language, using a mask other than the classful default to subdivide address space is the VLSM concept. Options A and B are unrelated: APIPA is the 169.254.0.0/16 link-local range, and RFC 1918 covers private IPv4 ranges such as 10.0.0.0/8 and 172.16.0.0/12. Neither describes changing a subnet mask.

Exam takeaway Count the borrowed bits first. /16 to /17 is one borrowed bit and two subnets. Then eliminate answers that describe address classes or special-use ranges rather than the act of subnetting. Correct answer: C.

Official Reference

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

Exam Strategy

When a scenario shows a mask longer than the classful default, count the borrowed bits and resulting subnets before scanning the answers. Keep address class separate from mask length: the first octet defines Class B or C, while /17 defines the subnet boundary. Eliminate APIPA and RFC 1918 choices unless the stem mentions 169.254 link-local or private ranges. This pattern usually points to VLSM.

Frequently Asked Questions

Does a 255.255.128.0 mask turn a Class B address into a Class C address?

No. The first octet still determines the address class, so a Class B address remains Class B. The /17 mask only changes the subnet boundary and creates two /17 subnets from the original /16 block.

Why is this change identified as VLSM rather than ordinary subnetting?

CompTIA uses VLSM for using a non-default mask to divide address space. Here the Class B default /16 is replaced by /17, borrowing one host bit; the same principle supports different subnet sizes across a network.

How many subnets and usable hosts result from a /16-to-/17 split?

Borrowing 1 bit creates 2^1 = 2 subnets. Each /17 subnet has 2^15 - 2 = 32,766 usable host addresses.

More N10-009 FAQ →

Community Votes

C
100%

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

Discussion 1 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 1

  1. ERB14 👍 1 Answered C
    VLSM (Variable Length Subnet Masking): Allows network administrators to divide an IP address space into subnets of variable sizes by changing the default classful subnet mask. By changing the standard Class B subnet mask of 255.255.0.0 (/16) to 255.255.128.0 (/17), 1 bit is borrowed from the host portion to split the single network into two smaller subnets

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