?– Using 4 bits for the subnet ?¬?eld results in 16 possible subnets (24 = 16).
?– Using 5 bits for the subnet ?¬?eld results in 32 possible subnets (25 = 32).
?– Using 6 bits for the subnet ?¬?eld results in 64 possible subnets (26 = 64).
In general, you can use the following formula to calculate the number of usable subnets, given the
number of subnet bits used:
Number of subnets = 2s (in which s is the number of subnet bits)
For example, you can subnet a network with a private network address of 172.16.0.0/16 so that it
provides 100 subnets and maximizes the number of host addresses for each subnet. The following
list highlights the steps required to meet these needs:
?– How many bits will need to be borrowed?
??” 2s = 27 = 128 subnets (s = 7 bits)
?– What is the new subnet mask?
??” Borrowing 7 host bits = 255.255.254.0 or /23
?– What are the ?¬?rst four subnets?
??” 172.16.0.0, 172.16.2.0, 172.16.4.0, and 172.16.6.0
Implementing Variable-Length Subnet Masks 125
?– What are the ranges of host addresses for the four subnets?
??” 172.16.0.1??“172.16.1.254
??” 172.16.2.1??“172.16.3.254
??” 172.16.4.1??“172.16.5.254
??” 172.16.
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