Question

You own a Class B network and need at least 29 subnets each with at least 1050 usable host IPs per subnet. What is your mask?

Answer 1

Since the beginning and ending IP-addresses of a subnet are generally reserved for broadcast addresses, so let's say that we require 1050 + 2 = 1052 addresses per subnet.

Thus, the number of bits needed for the IP-addresses in each subnet

= ceiling ( log₂1052 )

= 11

Also, number of bits needed to specify 29 subnets

= ceiling ( log₂29 )

= 5

Thus, total number of bits that we need to reserve for subnetting

= 11 + 5

= 16

Thus, the mask (for overall Class B network) will have lower 16 bits set to 0, and is given by

= binary 11111111.11111111.00000000.00000000

= decimal 255.255.0.0