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Table 3.3 describes how you can divide a Class C network into four equally sized subnets with a subnet mask of 255.255.255.192. This gives you 61 IP addresses on each subnet once you have accounted for the network, router, and broadcast default addresses.


TABLE 3.3 Class C Network Divided into Four Subnets


Network Number


First Address


Broadcast Address


X.Y.Z.0


X.Y.Z.1


X.Y.Z.63


X.Y.Z.64


X.Y.Z.65


X.Y.Z.127


X.Y.Z.128


X.Y.Z.129


X.Y.Z.191


X.Y.Z.192


X.Y.Z.193


X.Y.Z.255

Table 3.4 describes how you can divide a Class C network into eight equally sized subnets with a subnet mask of 255.255.255.224. This gives you 29 IP addresses on each subnet once you have accounted for the network, router, and broadcast default addresses.


TABLE 3.4 Class C Network Divided into Eight Subnets


Network Number


First Address


Broadcast Address


X.Y.Z.0


X.Y.Z.1


X.Y.Z.31


X.Y.Z.32


X.Y.Z.33


X.Y.Z.63


X.Y.Z.64


X.Y.Z.65


X.Y.Z.95


X.Y.Z.96


X.Y.Z.97


X.Y.Z.127


X.Y.Z.128


X.Y.Z.129


X.Y.Z.159


X.Y.Z.160


X.Y.Z.161


X.Y.Z.191


X.Y.Z.192


X.Y.Z.193


X.Y.Z.223


X.Y.Z.224


X.Y.Z.225


X.Y.Z.255

Classless Internetwork Domain Routing (CIDR)


InterNIC no longer gives out addresses under the Class A, B, or C designations. Instead, it uses a method called Classless Internetwork Domain Routing (or CIDR, which is usually pronounced “cider”). CIDR networks are described as “slash x” networks; the x represents the number of bits in the IP address range that InterNIC controls. This allows InterNIC to define networks that fall between the old classifications, which means that you can get a range of addresses much better suited to your needs than in times past. In CIDR terms, a network classified as a Class C network under the old scheme becomes a slash 24 network, because InterNIC controls the leftmost 24 bits and you control the rightmost 8 bits. Table 3.5 shows some examples of slash x network types.

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