Extending IP Addresses

Extending IP Addresses
In the “old days,” when the Network Information Center (InterNIC) assigned a network number to
an organization, it assigned either the first octet (a Class A network), the first two octets (a Class B
network), or the first three octets (a Class C network). The organization could take this one network
number and further subdivide it into smaller networks through a process called
subnetting
.
To illustrate, let’s say that our organization has been assigned the Class B network 172.16.0.0.
We have several different network segments, each of which needs a unique network number. So,
we decide to subnet our network. We use a subnet mask of 255.255.255.0. The
subnet mask
determines where in our IP address is the boundary between the subnet number and the host ID.
If we write our IP address and subnet mask out in binary, as illustrated in the following IP address
example, the 1s in the mask correspond to the network portion of the address, and the 0s correspond
to the node portion of the address.
So, in this case, instead of having one network (172.16.0.0) with 65,534 available host numbers,
we have 256 networks (172.16.0.0–172.16.255.0) with 254 available host numbers in each subnet.
We can calculate the number of hosts available on a subnet by using the formula 2
n
– 2
=
number of available host IPs
, where
n
is the number of host bits (in our example, 8). The minus
2 (– 2) represents our not being able to assign all host bits on and all host bits off, which are
reserved to an interface as an IP address.
Similarly, the number of networks (or subnets) can be calculated with nearly the same formula:
2
n
=
number of available networks
, where
n
is the number of subnet bits (in our example, 8). As long
as we use the
ip subnet-zero
global configuration command, we no longer need to subtract 2
from this result and may use the all 0s and all 1s subnets. Without this command configured, we
must also subtract 2 from this result, leaving only 254 subnets, not including subnet 0 and subnet 255.
So, with subnetting we have balanced our need for available network and host numbers. However,
there may be instances where we need fewer host numbers on a particular subnet and more host
numbers on another.
Decimal 172 16 0 0
Binary 10101100 00010000 00000000 00000000
Decimal 255 255 255 0
Binary 11111111 11111111 11111111 00000000