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The Actual Class A B and C IP Networks

Figure 4-7 shows the number of Class A, B, and C IP networks in the entire world.
Eventually, you need to actually pick and use some of these IP networks to build a working
TCP/IP internetwork, so you need to be able to answer the question: What are the specific
IP networks?

First, you must be able to identify each network briefly using a network identifier (network
ID). The network ID is just one reserved DDN value per network that identifies the IP network.
(The network ID cannot be used by a host as an IP address.) For example, Table 4-2
shows the network IDs that match the earlier Figure 4-5.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
NOTE Many people use the term network ID, but others use the terms network number
and network address. Be ready to use all three terms.

So, what are the actual Class A, B, and C IP networks, and what are their network IDs?
First, consider the Class A networks. Per Figure 4-7, only 126 Class A networks exist. As it
turns out, they consist of all addresses that begin with 1, all addresses that begin with 2, all
addresses that begin with 3, and so on, up through the 126th such network of “all addresses
that begin with 126.” Table 4-3 lists a few of these networks.
Free CISCO CCNA Routing and Switching ICND1 Study Guide

Class B networks have a first octet value between 128 and 191, inclusive, but in a single Class
B network, the addresses have the same value in the first two octets. For example, Figure 4-5
uses Class B network 130.4.0.0. The DDN value 130.4.0.0 must be in Class B, because the first
octet is between 128 and 191, inclusive. However, the first two octets define the addresses in
a single Class B network. Table 4-4 lists some sample IPv4 Class B networks.

Free CISCO CCNA Routing and Switching ICND1 Study Guide

Class C networks can also be easily identified, with a first octet value between 192 and 223,
inclusive. With Class C networks and addresses, the first three octets define the group, with
addresses in one Class C network having the same value in the first three octets. Table 4-5
shows some samples.

Free CISCO CCNA Routing and Switching ICND1 Study Guide

Listing all the Class A, B, and C networks would of course take too much space. For study
review, Table 4-6 summarizes the first octet values that identify the class and summarizes
the range of Class A, B, and C network numbers available in the entire IPv4 address space.

Free CISCO CCNA Routing and Switching ICND1 Study Guide

NOTE The term classful IP network refers to any Class A, B, or C network, because it is
defined by Class A, B, and C rules.

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