Information Technology Reference
In-Depth Information
The IP address is 32 bits long and can address over four billion physical addresses (2
32
or
4 294 967 296 hosts). There are three main address formats and these are shown in Figure
23.7.
0
Class A
Network (7 bits)
Node (24 bits)
1
0
Class B
Network (14 bits)
Node (16 bits)
1
1
0
Class C
Network (21 bits)
Node (8 bits)
Figure 23.7
Type A, B and C IP address classes
Each of these types is applicable to certain types of networks. Class A allows up to 128
(2
7
) different networks and up to 16 777 216 (2
24
) hosts on each network. Class B allows up
to 16 384 (2
14
) networks and up to 65 536 (2
16
) hosts on each network. Class C allows up to
2 097 152 (2
21
) networks each with up to 256 (2
8
) hosts.
The class A address is thus useful where there are a small number of networks with a
large number of hosts connected to them. Class C is useful where there are many networks
with a relatively small number of hosts connected to each network. Class B addressing gives
a good compromise of networks and connected hosts.
When selecting internet addresses for the network, the address can be specified simply
with decimal numbers within a specific range. The standard DARPA IP addressing format is
of the form:
W.X.Y.Z
where
W
,
X
,
Y
and
Z
represent 1 byte of the IP address. As decimal numbers they range from
0 to 255. The 4 bytes together represent both the network and host address.
The valid range of the different IP addresses is given in Figure 23.7 and Table 23.2 de-
fines the valid IP addresses. Thus for a class A type address there can be 127 networks and
1 6 711 680 (256
256
255) hosts. Class B can have 16 320 (64
255) networks and class C
×
×
×
can have 2 088 960 (32
×
256
×
255) networks and 255 hosts.
Addresses above
223.255.254
are reserved, as are addresses with groups of zeros.
Table 23.2
Ranges of addresses for type A, B and C internet address
Type
Network portion
Host portion
A
1 - 126
0.0.1 - 255.255.254
B
128.1 - 191.254
0.1 - 255.254
C
192.0.1 - 223.255.254
1 - 254
23.6.3 Creating IP addresses with subnet numbers
Besides selecting IP addresses of internets and host numbers, it is also possible to designate
an intermediate number called a subnet number. Subnets extend the network field of the IP
