With
a single bit pattern, you can produce two unique bit patterns, 1 and 0. If you
borrow 2 bits, you can provide four unique patterns to represent networks 00,
01, 10, and 11. Three bits would allow eight blocks, and so on.
Determine the
Total Number of Hosts
Recall
from the previous section that as you divide the address range into subnets,
you lose two host addresses for each new network. These are the network address
and broadcast address.
The formula
for calculating the number of hosts in a network is
Usable hosts = 2n – 2
where n
is the number of bits remaining to be used for hosts.
Subnetting a
Subnet
Subnetting
a subnet, or using variable-length subnet mask (VLSM), was designed to
maxi-mize addressing efficiency. VLSM is a practice associated with classless
addressing. When identifying the total number of hosts using traditional
subnetting, you allocate the same number of addresses for each subnet. If all
the subnets have the same requirements for the number of hosts, these
fixed-size address blocks would be efficient. However, that is most often not
the case.
For
example, the topology in Figure 6-24 shows a subnet requirement of seven
subnets, one for each of the four LANs and one for each of the three WANs. With
the given address of 192.168.20.0, you need to borrow 3 bits from the host bits
in the last octet, which provides eight subnets, to meet your subnet
requirement of seven subnets.
Figure
6-24 VLSM
Subnetting
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192.168.20.32/27
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192.168.20.0/27
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192.168.20.196/30
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192.168.20.200/30
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Building A
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Building
B
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Building C
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Building
D
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192.168.20.192/30
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192.168.20.96/27
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192.168.20.64/27
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Subnet Number
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Subnet
Address
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Subnet
Number
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Subnet
Address
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Subnet 0
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192.168.20.0/27
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Subnet 0
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192.168.20.192/30
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Subnet 1
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192.168.20.32/27
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Subnet 1
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192.168.20.196/30
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Subnet 2
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192.168.20.64/27
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Subnet 2
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192.168.20.200/30
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Subnet 3
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192.168.20.96/27
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Subnet 3
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192.168.20.204/30
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Subnet 4
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192.168.20.128/27
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Subnet 4
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192.168.20.208/30
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Subnet 5
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192.168.20.160/27
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Subnet 5
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192.168.20.212/30
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Subnet 6
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192.168.20.192/27
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Subnet 6
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192.168.20.216/30
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Subnet 7
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192.168.20.224/27
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Subnet 7
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192.168.20.20/30
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