Novell LINUX ENTERPRISE SERVER 11 - ADMINISTRATION Administration Manual page 217

17.1.1 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 17.1, "Writing IP
Addresses" (page 203).
Example 17.1 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
to 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 proved too inflexible and
was discontinued. Now, classless routing (CIDR, classless interdomain routing) is used.
17.1.2 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 for the subnetwork.
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 addresses are in the same local net-
work. 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 17.2, "Linking IP Addresses
to the Netmask" (page 204). 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 17.2, "Linking IP Addresses
to the Netmask" (page 204) the first net with 24 bits could also be written as
192.168.0.0/24.
Basic Networking 203