Novell LINUX ENTERPRISE SERVER 10 - INSTALLATION AND ADMINISTRATION 08-05-2008 Installation Manual page 565

30.1.1 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 30.1, "Writing IP
Addresses" (page 547).
Example 30.1 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.
30.1.2 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 30.2, "Linking IP Addresses
to the Netmask" (page 548). The netmask consists of 32 bits that identify how much of
an IP address belongs to the network. All those bits that are 1 mark the corresponding
bit in the IP address as belonging to the network. All bits that are 0 mark bits inside
the subnetwork. This means that the more bits are 1, the smaller the subnetwork is.
Because the netmask always consists of several successive 1 bits, it is also possible to
just count the number of bits in the netmask. In Example 30.2, "Linking IP Addresses
to the Netmask" (page 548) the first net with 24 bits could also be written as
192.168.0.0/24.
Basic Networking 547