Information Technology Reference
In-Depth Information
address beyond the limit defined by the type A, B, C scheme. They allow a hierarchy of
internets within a network. For example, it is possible to have one network number for a
network attached to the internet, and various subnet numbers for each subnet within the net-
work. This is illustrated in Figure 23.8.
Network number
Subnet number
Host number
Su
bn
et A
[S
ubnet
number]
Net
work A
[
Ne
twork
nu
mber]
Su
bnet A
[Subnet
number]
Net
wo
rk C
[N
etwork
nu
mb
er]
Su
bnet B
[Subnet
number]
Su
b
net B
[Subnet
number]
W.X.Y
Su
b
net C
[Subnet
number]
W.X.Y
Figure 23.8
IP addresses with subnets
For an address
W.X.Y.Z
for a type A address
W
specifies the network and
X
the subnet.
For type B the
Y
field specifies the subnet, as illustrated in Figure 23.9.
To connect to a global network a number is normally assigned by a central authority. For
the Internet network it is assigned by the network information center (NIC). Typically, on the
Internet an organisation is assigned a type B network address. The first two fields of the ad-
dress specify the organisation network, the third specifies the subnet within the organization
and the final value specifies the host.
Network number
(W.X)
Subnet number
(Y)
Host number
(Z)
W.X.Y
Sub
n
et A
[S
ubnet
number]
W.X
W.X.Y.Z
Netw
ork A
[
N
etwork
nu
mber]
W.X
Su
bnet A
[Subnet
number]
Net
wo
rk C
[N
et
w
ork
nu
mb
er]
Su
bnet B
[Subnet
number]
Su
b
net B
[Subnet
number]
W.X.Y.Z
W.X.Y
Su
b
net C
[Subnet
number]
W.X.Y
W.X.Y
Figure 23.9
Internet addresses with subnets
