Network Addressing
An IP address is a
numeric identifier assigned to each machine on an IP network. It designates the
specific location of a device on the network.
Terminology:
* Bit - A bit is one digit, either a 1 or a
0.
* Octet - An octet, made up of 8 bits, is
just an ordinary 8-bit binary number.
* Network address - This is the designation
used in routing to send packets to a remote network - for example, 10.0.0.0,
172.16.0.0, and 192.168.1.0.
* Broadcast address - The address used by
applications and hosts to send information to all nodes on a network is called
the broadcast address. Examples include 255.255.255.255, which is all networks,
all nodes; 172.16.255.255, which is all subnets and hosts on network
172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on
network 10.0.0.0.
An IP address consists
of 32 bits of information. These bits are divided into four sections, referred
to as octets or bytes, each containing 1 byte (8 bits). You can depict an IP
address using one of three methods:
* Dotted-decimal, as in 172.20.18.125
* Binary, as in
10101100.00010100.00010010.01111101
The 32-bit IP address
is a structured or hierarchical address, as opposed to a flat or
nonhierarchical address. Although either type of addressing scheme could have
been used, hierarchical addressing was chosen for a good reason. The advantage
of this scheme is that it can handle a large number of addresses, namely 4.3
billion (a 32-bit address space with two possible values for each position -
either 0 or 1 - gives you 232, or 4,294,967,296).
The network address
(which can also be called the network number) uniquely identifies each network.
Every machine on the same network shares that network address as part of its IP
address. In the IP address 172.20.18.125, for example, 172.20 is the network
address. The node address is assigned to, and uniquely identifies, each machine
on a network. This part of the address must be unique because it identifies a
particular machine - an individual - as opposed to a network, which is a group.
This number can also be referred to as a host address. In the sample IP address
172.20.18.125, the 18.125 is the node address. The designers of the Internet
decided to create classes of networks based on network size. For the small
number of networks possessing a very large number of nodes, they created the rank
Class A network. At the other extreme is the Class C network, which is reserved
for the numerous networks with a small number of nodes. The class distinction
for networks between very large and very small is predictably called the Class
B network.
Class A
|
Class
B
|
Class
C
|
|
First Octet Range
|
1 to 126
|
128 to 191
|
192 to 223
|
Valid Network Numbers
|
1.0.0.0 to 126.0.0.0
|
128.1.0.0 to 191.254.0.0
|
192.0.1.0 to 223.255.254.0
|
Number of Networks in This Class
|
27 - 2
|
214 - 2
|
221 - 2
|
Number of Hosts per Network
|
224 - 2
|
216 - 2
|
28 - 2
|
Size of Network Part of Address
(bytes/bits)
|
1 / 8
|
2 / 16
|
3 / 24
|
Size of Host Part of Address
(bytes/bits)
|
3 / 24
|
2 / 16
|
1 / 8
|
Default Mask
|
255.0.0.0
|
255.255.0.0
|
255.255.255.0
|
By definition, an IP
address that begins with 8 in the first octet is in a Class A network, so the
network part of the address is the first byte, or first octet. An address that
begins with 130 is in a Class B network. By definition, Class B addresses have
a 2-byte network part, as shown. Finally, any address that begins with 199 is
in a Class C network, which has a 3-byte network part. Also by definition, a
Class A address has a 3-byte host part, Class B has a 2- byte host part, and
Class C has a 1-byte host part.
IP Subnetting
IP subnetting creates
vastly larger numbers of smaller groups of IP addresses compared with simply
using Class A, B, and C conventions. The Class A, B, and C rules still exist,
but now a single Class A, B, or C network can be subdivided into many smaller groups.
Subnetting treats a subdivision of a single Class A, B, or C network as if it
were a network itself. By doing so, a single Class A, B, or C network can be
subdivided into many nonoverlapping subnets. Fig. 13 and Fig. 14 show the basic
differences between a network that does not use subnetting and one that does.
First, look at Fig. 13, which uses six different IP networks.
The design shown in
Fig. 13 requires six groups, each of which is a Class B network. The four LANs
each use a single Class B network. In other words, the LANs attached to Routers
A, B, C, and D are each a separate network. Additionally, the two serial
interfaces comprising the point-to-point serial link between Routers C and D
use the same network, because these two interfaces are not separated by a
router. Finally, the three router interfaces comprising the Frame Relay network
with Routers A, B, and C are not separated by an IP router and would comprise
the sixth network.
Fig-13
Fig. 14 uses six
subnets, each of which is a subnet of a single Class B network. This design
subnets Class B network 160.30.0.0. The IP network designer has chosen a mask
of 255.255.255.0, the last octet of which implies 8 host bits. Because it is a
Class B network, there are 16 network bits. Therefore, there are 8 subnet bits,
which happen to be bits 17 through 24-in other words, the third octet.
Fig -14
Note that the network
parts (the first two octets in this example) all begin with 150.150, meaning
that each of the six subnets is a subnet of Class B network 150.150.0.0.
With subnetting, the
third part of an IP address-namely, the subnet-appears in the middle of the
address. This field is created by “stealing” or “borrowing” bits from the host
part of the address. The size of the network part of the address never shrinks.
In other words, Class A, B, and C rules still apply when you define the size of
the network part of an address. However, the host part of the address shrinks
to make room for the subnet part of the address. Fig. 15 shows the format of
addresses when subnetting is used.
Fig-14
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