A Class B network with default mask /16 is subnetted using the mask /20.…
2023
A Class B network with default mask /16 is subnetted using the mask /20. Assuming modern subnetting rules where subnet-zero is allowed, how many subnets are created?
- A.
8
- B.
16
- C.
32
- D.
64
- E.
256
Attempted by 10 students.
Show answer & explanation
Correct answer: B
Concept
When a network is subnetted, the longer subnet mask borrows bits from the host portion and converts them into subnet bits. The number of subnets created equals 2 raised to the number of borrowed bits, where borrowed bits = (new prefix length) - (original prefix length). When subnet-zero is allowed, the all-zeros and all-ones subnets are both counted, so nothing is subtracted from the total.
Application
Original prefix from the default Class B mask is /16.
The new subnetting mask is /20.
Borrowed (subnet) bits = 20 - 16 = 4.
Number of subnets = 24 = 16.
Cross-check
Doubling for each borrowed bit gives 2, 4, 8, 16 for 1, 2, 3, 4 bits respectively, so 4 borrowed bits yield 16. Since subnet-zero is allowed, every one of these 16 blocks is usable, confirming the count is 16.