This kind of address is still in wide use and is what people commonly refer to as an IP address. Part of an IP address is used for the network address, and the other part is used to identify a particular interface on a host in that network. You should realize that IP addresses are assigned to interfaces-such as Ethernet cards or modems-and not to the host computer. Usually a computer has only one interface and is accessed using only that interface's IP address. In that regard, an IP address can be thought of as identifying a particular host system on a network, and so the IP address is usually referred to as the host address.
In fact, though, a host system could have several interfaces, each with its own IP address. This is the case for computers that operate as gateways and firewalls from the local network to the Internet. One interface usually connects to the LAN and another to the Internet, as by two Ethernet cards. Each interface (such as an Ethernet card) has its own IP address. For example, when you use the Red Hat Network Configuration tool to specify an IP address for an Ethernet card on your system, the Devices panel lists an entry for each Ethernet card installed on your computer, beginning with eth0 for the first. Opening up a Device window, you can select the TCP protocol in the Protocols panel to open a TCP/IP setting window where you can enter the card's IP address. Other Ethernet cards have their own IP addresses. Currently, the Linux kernel can support up to four network adapters. If you use a modem to connect to an ISP.
Originally, IP addresses were organized according to
classes. On the Internet, networks are organized into three classes depending on
their size-classes A, B, and C. A class A network uses only the first segment
for the network address and the remaining three for the host, allowing a great
many computers to be connected to the same network. Most IP addresses reference
smaller, class C, networks. For a class C network, the first three segments are
used to identify the network, and only the last segment identifies the host.
Altogether, this forms a unique address with which to identify any network
interface on computers in a TCP/IP network. For example, in the IP address
192.168.1.72, the network part is 192.168.1 and the interface/host part is 72.
The interface/host is a part of a network whose own address is 192.168.1.0.
In a class C network, the first three numbers identify the network
part of the IP address. This part is divided into three network numbers, each
identifying a subnet. Networks on the Internet are organized into subnets,
beginning with the largest and narrowing to small subnetworks. The last number
is used to identify a particular computer, referred to as a host. You can think of the Internet as a series of networks
with subnetworks; these subnetworks have their own subnetworks. The rightmost
number identifies the host computer, and the number preceding it identifies the
subnetwork of which the computer is a part. The number to the left of that
identifies the network the subnetwork is part of, and so on. The Internet
address 192.168.187.4 references the fourth computer connected to the network
identified by the number 187.
An IPv6 address is written as 8 segments representing 16 bits each 128 bits total. To more easily represent 16 bit binary numbers, hexadecimal
numbers are used. Hexadecimal numbers use 16 unique numbers, instead of the 8
used in octal numbering. These are 0 through 9, continuing with the characters A
through F.
In the following example, the first four segments represent the
network part of the IPv6 address, and the following four segments represent the
interface (host) address.
FEC0:0000:0000:0000:0008:0800:200C:417A
You can cut any preceding zeros, but not trailing zeros in any
given segment. Segments with all zeros can be reduced to a single zero.
FEC0:0:0:0:8:800:200C:417A
The loopback address used for locahost addressing can be written
with seven preceding zeros and a 1.
0:0:0:0:0:0:0:1
Many addresses will have sequences of zeros. IPv6 supports a
shorthand symbol for representing a sequence of several zeros in adjacent
fields. This consists of a double colon ::. There can be only one use of the ::
symbol per address.
FEC0::8:800:200C:417A
