242
A
PPENDIX
C: A
DRESSING
S
CHEMES
In the figure below, notice that the position of this line is determined by
the position of the first zero bit in the address.
Figure 1
Address Class Map
Subnetting
A large IP network can be subdivided into smaller subnetworks. This is
done using a subnet mask (in this text, often called netmask), which tells
a routing device how to further subdivide the Host ID portion of an IP
address.
A subnet mask is a 32 bit value which is written in dotted decimal
notation. It contains a number of bits set to 1 (indicating the network
portion of an address) followed by a number of bits set to 0 (indicating
the host portion of an address).
For example, a netmask of 255.255.255.0 on a Class B network indicates
that the network is divided into 254 subnetworks of 254 nodes each (0
and 255 are reserved numbers). 128.5.63.28 is host 28 on subnetwork
63 of that network. The natural network itself is 128.5.0.0 (Class B
network).
Notice that by using subnet masks, you can define a natural hierarchy in
which the addresses themselves indicate how a packet is to be routed.
But, all routing devices on an IP network must be using the same
subnetting scheme.
Also note that a subnet mask for a given network segment is not part of
the address and is not transmitted with every packet. It is simply a value
which is known to all the routing devices adjacent to that segment.
CLASS A
CLASS B
CLASS C
0
0
0
1
1 1
NET ID
NET ID
NET ID
0 1 2 3 4
8
16
24
31
HOST ID
HOST ID
HOST ID
