Information Technology Reference
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as part of its IP address, just as the address of every house on a street shares the same street
name. In the IP address 130.57.30.56, for example, 130.57 is the network address.
The node address—the second two sets of numbers—is assigned to, and uniquely
identifies, each machine in a network, just as each house on the same street has a different
house number. This part of the address must be unique because it identifies a particular
machine—an individual, as opposed to a network. This number can also be referred to as a
host address
. In the sample IP address 130.57.30.56, the node address is .30.56.
Understanding Network Classes
The designers of the Internet decided to create classes of networks based on network size.
For the small number of networks possessing a very large number of nodes, they created the
Class A network. At the other extreme is the Class C network, reserved for the numerous
networks with small numbers of nodes. The class of networks in between the very large
and very small ones is predictably called the Class B network.
The default subdivision of an IP address into a network and node address is determined
by the class designation of your network. Table 8.2 summarizes the three classes of
networks, which will be described in more detail in the following sections.
Table 8.2
Network address classes
Leading bit
pattern
Decimal range of first
octet of IP address
Assignable
networks
Maximum nodes
per network
Class
Mask bits
A
8
0
1-126
126
16,777,214
B
16
10
128 -191
16,384
65,534
C
24
110
192-223
2,097,152
254
Classless Inter-Domain Routing (CIDR), explained in detail later in this
chapter, has effectively done away with these class designations. You will
still hear and should still know the meaning behind the class designations
of addresses because they are important to understanding IP addressing.
However, when you're working with IP addressing in practice, CIDR is more
important to know.
To ensure efficient routing, Internet designers defined a mandate for the leading bits
section of the address for each different network class. For example, because a router knows
that a Class A network address always starts with a 0, it can quickly apply the default mask,
if necessary, after reading only the first bit of the address. Table 8.2 illustrates how the
leading bits of a network address are defined. When considering the subnet masking between
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