A single Class B network (default mask /16) is subnetted using the subnet mask 255.255.240.0 (/20).
How many subnets are created from this network? 

Correct option is C

A Class B network has a default subnet mask of /16, meaning 16 bits are used for the network portion.

The new subnet mask given is /20.

Number of bits borrowed for subnetting = 20 − 16 = 4 bits

The number of subnets created is calculated using:$$2^ {number of borrowed bits}=2^4=16$$​

Therefore, 16 subnets are created. Hence, the correct answer is (b).

Important Key Points:

1. A Class B network has a default mask of /16.
2. Subnetting from /16 to /20 means 4 bits are borrowed.
3. Number of subnets = 2ⁿ, where n = number of borrowed bits.
4. Here, 2⁴ = 16 subnets are formed.

Knowledge Booster:

• 8: Incorrect because 8 = 2³, but 4 bits are borrowed, not 3.
• 32: Incorrect because 32 = 2⁵, which would require 5 borrowed bits.
• 64: Incorrect because 64 = 2⁶, which would require 6 borrowed bits.
• 256: Incorrect because 256 = 2⁸, which would require 8 borrowed bits.