Because of the physical and architectural limitations on the size of networks, you often must break large networks into smaller subnetworks. Within a network, each wire or ring requires its own network number and identifying subnet address.
Figure 9 shows two subnets in a network.
Figure 9: Subnets in a Network
Figure 9 shows three devices connected to one subnet and three more devices connected to a second subnet. Collectively, the six devices and two subnets make up the larger network. In this example, the network is assigned the network prefix 192.14.0.0, a class B address. Each device has an IP address that falls within this network prefix.
In addition to sharing a network prefix (the first two octets), the devices on each subnet share a third octet. The third octet identifies the subnet. All devices on a subnet must have the same subnet address. In this case, the alpha subnet has the IP address 192.14.126.0 and the beta subnet has the IP address 192.14.17.0.
The subnet address 192.14.17.0 can be represented as follows in binary notation:
11000000 . 00001110 . 00010001 . xxxxxxxx
Because the first 24 bits in the 32-bit address identify the subnet, the last 8 bits are not significant. To indicate the subnet, the address is written as 192.14.17.0/24 (or just 192.14.17/24). The /24 is the subnet mask (sometimes shown as 255.255.255.0).
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