Internet Protocol
Classes - Network & Host ID
Introduction
Every protocol suite defines some type of addressing
that identifies computers and networks. IP Addresses
are no exception to this "rule". There are certain
values that an IP Address can take and these have
been defined by the IEEE committee (as most things).
A simple IP Address is a lot more than just a
number. It tells us the network that the workstation
is part of and the node ID. If you don't understand
what I am talking about, don't let it worry you too
much because we are going to analyse everything here
:)
IP Address Classes and Structure
When the IEEE committee sat
down to sort out the range of numbers that were
going to be used by all computers, they came out
with 5 different ranges or, as we call them, "Classes"
of IP Addresses and when someone applies for IP
Addresses they are given a certain range within a
specific "Class"
depending on the size of their network.
To keep things as simple as possible, let's first
have a look at the 5 different Classes:
In the above table, you can
see the 5 Classes.
Our first Class is
A
and our last is E.
The first 3 classes ( A,
B
and C)
are used to identify workstations, routers, switches
and other devices whereas the last
2 Classes
( D
and
E)
are reserved for special use.
As you would already know an IP Address consists of
32 Bits, which means it's 4 bytes long. The first
octec (first 8 Bits or first byte) of an IP Address
is enough for us to determine the Class to which it
belongs. And, depending on the Class to which the IP
Address belongs, we can determine which portion of
the IP Address is the Network ID and which is the
Node ID.
For example, if I told you
that the first octec of an IP Address is "168"
then, using the above table, you would notice that
it falls within the 128-191
range, which makes it a
Class B
IP Address.
Understanding the Classes
We are now going to have a
closer look at the 5 Classes. If you remember
earlier I mentioned that companies are assigned
different IP ranges within these classes, depending
on the size of their network. For instance, if a
company required 1000 IP Addresses it would probably
be assigned a range that falls within a
Class B
network rather than a Class
A
or
C.
The
Class A
IP Addresses were designed for
large networks, Class B
for medium size networks and
Class C
for smaller networks.
Introducing Network ID and Node ID concepts
We need to understand the Network ID and Node ID
concept because it will help us to fully understand
why Classes exist. Putting it as simply as possible,
an IP Address gives us 2 pieces of valuable
information:
1)
It tells us which network the device is part of (Network
ID).
2)
It identifies that unique
device within the network (Node
ID).
Think of the Network ID as the suburb you live in
and the Node ID your street in that suburb. You can
tell exactly where someone is if you have their
suburb and street name. In the same way, the Network
ID tells us which network a particular computer
belongs to and the Node ID identifies that computer
from all the rest that reside in the same network.
The picture below gives you a small example to help
you understand the concept:
Explanation:
In the above picture, you can
see a small network. We have assigned a Class C IP
Range for this network. Remember that Class C IP
Addresses are for small networks. Looking now at
Host A, you will see that its IP Address is
192.168.0.2.
The
Network ID
portion of this IP Address is in
blue,
while the Host ID
is in orange.
Isuppose
the next question someone would ask is: How do I
figure out which portion of the IP Address is the
Network ID
and which is the
Host ID
?
That's what we are going to answer next.
The Network and Node ID of each Class
The network Class helps us determine how the 4 byte,
or 32 Bit, IP Address is divided between network and
node portions.
The table below shows you (in
binary) how the Network
ID and
Node ID
changes depending on the
Class:
Explanation:
The table above might seem confusing at first but
it's actually very simple. We will take Class A as
an example and analyse it so you can understand
exactly what is happening here:
Any
Class A
network has a total of
7 bits
for the Network ID
(bit
8 is always set to 0)
and 24 bits for the Host ID. Now all we need to do
is calculate how much 7 bits is:
2 to the power of 7
=
128 Networks
and for the hosts : 2
to the power of 24 =
16,777,216 hosts
in each
Network,
of which 2 cannot be used because one is the
Network Address and
the other is the
Network Broadcast address
(see the table towards the end
of this page). This is why when we calculate the
"valid" hosts in a network we always subtract "2".
So if I asked you how many "valid" hosts can you
have a on Class A Network, you should answer
16,777,214 and NOT 16,777,216.
Below you can see all this in one picture:
The same story applies for the
other 2 Classes we use, that's
Class B
and
Class C,
the only difference is that the number of networks
and hosts changes because the bits assigned to them
are different for each class.
Class B
networks have
14 bits
for the Network ID
(Bits 15, 16 are set and can't be changed) and
16 bits
for the
Host ID,
that means you can have up to '2
to the power of 14' =
16,384 Networks
and '2 to the power of
16' =
65,536 Hosts
in each Network,
of which 2 cannot be used because one is the
Network Address
and the other is the
Network Broadcast address
(see the table towards
the end of this page). So if I asked you how many
"valid" hosts can you have a on Class B Network, you
should answer 65,534 and NOT 65,536.
Class C
networks have 21 bits
for the
Network ID (Bits 22,
23, 24 are set and can't be changed) and
8 bits for the
Host ID, that means you
can have up to '2 to the power
of 21' = 2,097,152
Networks and '2 to the
power of 8' = 256 Hosts
in each Network, of
which 2 cannot be used because one is the
Network Address
and the
other is the Network Broadcast
address (see the table towards the end of
this page). So if I asked you how many "valid" hosts
can you have a on Class C Network, you should answer
254 and NOT 256.
Now, even though
we have 3 Classes
of IP
Addresses that we can use, there are some IP
Addresses that have been reserved for special use.
This doesn't mean you can't assign them to a
workstation but in the case that you did, it would
create serious problems within your network. For
this reason it's best that you avoid using these IP
Addresses.
The following
table shows the IP Addresses that you should avoid
using:
|
IP
Address
|
Function
|
|
Network 0.0.0.0
|
Refers to the default
route. This route is to simplify routing
tables used by IP.
|
|
Network 127.0.0.0
|
Reserved for Loopback. The Address
127.0.0.1 is often used to refer to the
local host. Using this Address,
applications can address a local host as
if it were a remote host.
|
| |
|
|
IP Address with all
host bits
set to "0" (Network
Address) e.g 192.168.0.0
|
Refers to the actual
network itself. For example, network
192.168.0.0 can be used to identify
network 192.168. This type of notation
is often used within routing tables.
|
|
IP Address with all
node bits
set
to "1" (Subnet
/ Network Broadcast) e.g 192.168.255.255
|
IP Addresses with all
node bits set to "1" are local
network broadcast addresses and must
NOT be used.
Some examples:
125.255.255.255
(Class A) ,
190.30.255.255
(Class B),
203.31.218.255
(Class C).
See "Multicasts"
& "Broadcasts"
for more info.
|
|
IP Address with
all bits
set to "1" (Network Broadcast) e.g
255.255.255.255
|
The IP Address with
all bits set to "1" is a broadcast
address and must NOT be used.
These are destined for all nodes on a
network, no matter what IP Address they
might have.
|
Now make sure you
keep to the above guidelines because you're going to
bump into a lot of problems if you don't !
IMPORTANT NOTE:
It is imperative that every network,
regardless of Class and size, has a
Network Address
(first IP Address e.g 192.168.0.0 for
Class C network) and a
Broadcast Address
(last IP Address e.g 192.168.0.255 for
Class C network), as mentioned in the
table and explanation diagrams above,
which cannot
be used.
So
when calculating available IP Addresses
in a network, always remember to
subtract 2 from the number of IP
Addresses within that network.
|
That all pretty
much covers this section.
Next, is the
Subnetting section, and before you proceed, make
sure you're comfortable with the new concepts and
material we have covered, otherwise subnetting will
be very hard to understand.
|
