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31.1 IP Addresses and Routing
The discussion in this section is limited to IPv4 networks. For information about IPv6
protocol, the successor to IPv4, refer to
Section 31.2, "IPv6—The Next Generation
Internet"
(page 568).
31.1.1 IP Addresses
Every computer on the Internet has a unique 32-bit address. These 32 bits (or 4 bytes)
are normally written as illustrated in the second row in
Example 31.1, "Writing IP
Addresses"
(page 565).
Example 31.1 Writing IP Addresses
IP Address (binary):
11000000 10101000 00000000 00010100
IP Address (decimal):
192.
168.
0.
20
In decimal form, the four bytes are written in the decimal number system, separated by
periods. The IP address is assigned to a host or a network interface. It cannot be used
anywhere else in the world. There are exceptions to this rule, but these are not relevant
in the following passages.
The points in IP addresses indicate the hierarchical system. Until the 1990s, IP addresses
were strictly categorized in classes. However, this system has proven too inflexible and
was discontinued. Now, classless routing (CIDR, classless interdomain routing) is used.
31.1.2 Netmasks and Routing
Netmasks are used to define the address range of a subnetwork. If two hosts are in the
same subnetwork, they can reach each other directly, if they are not in the same subnet-
work, they need the address of a gateway that handles all the traffic between the subnet-
work and the rest of the world. To check if two IP addresses are in the same subnet,
simply "AND" both addresses with the netmask. If the result is identical, both IP ad-
dresses are in the same local network. If there are differences, the remote IP address,
and thus the remote interface, can only be reached over a gateway.
To understand how the netmask works, look at
Example 31.2, "Linking IP Addresses
to the Netmask"
(page 566). The netmask consists of 32 bits that identify how much of
Basic Networking
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