Internet addresses are used extensively by LAN and WAN products
as well as Internet Services.
The network address identifies the network. The host address
identifies a node within the network. A network address is concatenated
with a host address to form the internet address and to uniquely
identify a node within a network.
Internet Address Formats |
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There are four internet address classes, each accommodating
a different number of network and host addresses. The address classes
are defined by the most significant bits of the binary form of the
address as shown in Figure 6-1 “Internet Address Classes”.
This section discusses three of the classes (Class A, Class B, Class
C). The fourth class (Class D) is discussed in the section "IP Multicast
Addresses."
The address classes can also be broken down by address ranges.
Internet addresses are typically represented by converting the bits
to decimal values an octet (8 bits) at a time, and separating each
octet's decimal value by a period ( . ). Therefore,
internet addresses are typically of the following form:
where n is a number from 0 to 255, inclusive. This form
is referred to as decimal dot notation or dot notation.
Table 6-4 “Internet Address Classes” lists the number
of networks and nodes and the address ranges for Class A, Class
B, and Class C networks. Class D networks are described later in
this chapter in "IP Multicast Addresses."
Table 6-4 Internet Address Classes
Class | Networks | Nodes
per Network | Address
Range |
|---|
A | 127 | 16777215 | 1.0.0.1 - 126.255.255.254 |
B | 16383 | 65535 | 128.1.0.1 - 191.255.255.254 |
C | 2097151 | 255 | 192.0.1.1 - 223.255.255.254 |
Reserved | - | - | 224.0.0.0 - 255.255.255.255 |
To determine a network address and host address from an internet address, you
must separate the network and host address fields. For example,
the bit representation of internet address 192.6.1.1 is separated
as follows:
Assigning an Internet Address |
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Each node on the network has at least one internet address.
When assigning internet addresses, you must determine network addresses and
host addresses as described in this section.
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 | NOTE: When specifying internet addresses, do not use leading
zeroes within address fields. For example: 192.006.012.023 is incorrect;
192.6.12.23 is correct. |
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Assigning Network Addresses
To assign network addresses, follow these rules:
You must have a network address for
each logical network.
If your system is attached to more than one physical
network via a gateway, the network addresses of these interfaces
may not be the same. Refer to Figure 6-3 “Assigning Network
Addresses” below for a gateway example.
All nodes in the same network, however, must have
the same network address.
Do not assign the network addresses 0 or 255 (Class
A), 0.0 or 255.255 (Class B), or 0.0.0 or 255.255.255 (Class C)
to any network. Those addresses are reserved .
Do not assign Class A network address 127. This
address is reserved for the loopback interface.
IP Address for Loopback Interface (lo0)
The loopback interface (lo0) is automatically configured when the system boots
with the TCP/IP software. The loopback interface is really a "pseudo-device," since
there is no hardware card associated with it. This interface is
created solely to facilitate sending IP datagrams to the IP loopback
address 127.0.0.1.
The default IP address and netmask of the loopback interface
are 127.0.0.1 and 255.0.0.0, respectively. By default, the loopback
interface sets up routing entries so that packets for any 127.*.*.*
address will loop back to the local host. Any attempt to change
the address of the initial loopback interface (lo0:0) will fail.
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