MCSE :
9. Finding the number of Hosts and subnets available for a
given
subnet mask: For example, let us find the number of hosts and
subnets available for an IP 156.233.42.56 with a subnet mask of 7
bits.
a. Class B network has the form N.N.H.H, the default subnet mask is
16 bits long. There is additional subnet mask of 7 bits long.
b. 7 bits of subnet
mask corresponds to (2^7-2)=128-2 = 126 subnets.
c. 9 bits (16-7) of
host addresses corresponds to (2^9-2)=512-2 = 510 hosts.
Some times, the
subnet mask is specified with the bits available in the default
subnet mask. In this case the bits available in default subnet mask
is 16. Therefore, total number of bits available in the subnet mask
are 16+7=23. If you are given a subnet mask of 23 bits long for a
class B address, it is understood that it contains the bits from the
default subnet mask as well.
Hence,
126 subnets and 510 hosts are available.
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