Operation Manual
9.1.1 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 9.1, “Writing IP Ad-
dresses” (page 155).
Example 9.1
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 can be used only
once throughout 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 inexible and
was discontinued. Now, classless routing (CIDR, classless interdomain routing) is used.
9.1.2 Netmasks and Routing
Netmasks are used to dene 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 trafc 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 9.2, “Linking IP Addresses to
the Netmask” (page 156). 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 9.2, “Linking IP Addresses to the Net-
mask” (page 156) the rst net with 24 bits could also be written as 192.168.0.0/24.
Basic Networking 155