Information Technology Reference
In-Depth Information
IP addressing requirements for each individual location
■
Number of devices in each communication closet
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Site requirements: VoIP devices, wireless LAN, video
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Subnet size
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The following example shows subnetting for a small company. Suppose the company has
200 hosts and is assigned the Class C network of 195.10.1.0/24. The 200 hosts need to be
in six different LANs.
Yo u c a n s u b n e t t h e C l a s s C n e t w o r k u s i n g a m a s k o f 2 5 5 . 2 5 5 . 2 5 5 . 2 2 4 . L o o k i n g a t t h e
mask in binary (11111111 11111111 11111111 11100000), the first 3 bytes are the net-
work part, the first 3 bits of the fourth byte determine the subnets, and the 5 remaining 0
bits are for host addressing.
Ta ble 8 -1 7 shows the subnetworks created with a mask of 255.255.255.224. Using this
mask, 2
n
subnets are created, where
n
is the number of bits taken from the host part for
the subnet mask. This example uses 3 bits, so 2
3
= 8 subnets. The first column of the table
lists the LANs. The second column shows the binary of the fourth byte of the IP address.
The third column shows the subnet number, and the fourth and fifth columns show the
first host and broadcast address of the subnet.
Ta b l e 8 -1 7
Subnets for Network 195.1.1.0
LAN
Fourth Byte
Subnet Number
First Host
Broadcast Address
LAN 0
00000000
195.10.1.0
195.10.1.1
195.10.1.31
LAN 1
00100000
195.10.1.32
195.10.1.33
195.10.1.63
LAN 2
01000000
195.10.1.64
195.10.1.65
195.10.1.95
LAN 3
01100000
195.10.1.96
195.10.1.97
195.10.1.127
LAN 4
10000000
195.10.1.128
195.10.1.129
195.10.1.159
LAN 5
10100000
195.10.1.160
195.10.1.161
195.10.1.191
LAN 6
11000000
195.10.1.192
195.10.1.193
195.10.1.223
LAN 7
11100000
195.10.1.224
195.10.1.225
195.10.1.255
Use the formula 2
n
- 2 to calculate the number of hosts per subnet, where
n
is the num-
ber of bits for the host portion. The preceding example has 5 bits in the fourth byte for
host addresses. With
n
= 5, 2
5
- 2 = 30 hosts. For LAN 1, host addresses range from
195.10.1.33 to 195.10.1.62 (30 addresses). The broadcast address for the subnet is
195.10.1.63. Each LAN repeats this pattern with 30 hosts in each subnet.
