Subnet Masks &
Their Effect
Introduction
There are a few different ways to approach
subnetting and it can get confusing because of the
complexity of some subnets and the flexibility they
offer. For this reason I created this little
paragraph to let you know how we are going to
approach and learn subnetting. So.....
We are going to analyse the common subnet masks for
each Class, giving detailed examples for most of
them and allowing you to "see" how everything is
calculated and understand the different effects a
subnet mask can have as you change it. Once you have
mastered this, you can then go on and create your
custom subnet masks using any type of Class.
Default Subnet masks of each Class
By now you should have some
idea what the subnet mask does and how it's used to
partition a network. What you need to keep in mind
is that each Class has its
DEFAULT
subnet mask,
which we can change to suit our needs. I have
already mentioned this in the previous page, but we
need to look into it in a bit more detail.
The picture below shows our 3 Network Classes with
their respective default subnet mask:
The Effect of a Subnet Mask on an IP Address
In the
IP Classes
page we analysed and showed
clearly how an IP Address consists of two parts, 1)
The Network ID
and 2)
The Host ID.
This rule applies for all IP
Addresses that use the default subnet mask and we
call them Classful IP
Addresses.
We can see this once again in the picture below,
where the IP Address is analysed in Binary, because
this is the way you should work when dealing with
subnet masks:
We are looking at an IP
Address with its subnet mask for the first time.
What we have done is take the decimal subnet mask
and converted it to binary, along with the IP
Address. It is essential to work in binary because
it makes things clearer and we can avoid making
silly mistakes. The
ones (1)
in the subnet mask "lock" or, if you like, define
the Network ID
portion. If we change any bit
within the Network ID
of the
IP Address,
then we immediately move to a different network. So
in this example, we have a
24 bit subnet mask.
NOTE:
All
Class C Classful IP Addresses
have a
24 bit subnet mask
(255.255.255.0).
All
Class B Classful IP Addresses
have a
16 bit subnet mask
(255.255.0.0).
All
Class A Classful IP Addresses
have an
8 bit subnet mask
(255.0.0.0).
On the other hand, the use of
an IP Address with a subnet mask other than the
default results in the standard
Host bits
(the Bits used to indentify
the HOST ID)
being divided in to two parts: a
Subnet ID
and
Host ID.
These type of IP Addresses are called
Classless IP Addresses.
In
order to understand what a "Classless
IP Address" is without
getting confused, we are going to take the same IP
Address as above, and make it a
Classless IP Address
by changing the default subnet
mask:
Looking at the picture above
you will now notice that we have a
Subnet ID,
something that didn't exist before. As the picture
explains, we have borrowed
3 bits
from the Host ID
and used them to create a
Subnet ID.
Effectively we partitioned our
Class C network into smaller networks.
If you're wondering how many smaller networks,
you'll find the answer on the next page. I prefer
that you understanding everything here rather than
blasting you with more Subnet ID's, bits and all the
rest :)
Summary
In this page we saw the default subnet mask of each
Class and also introduced the Classful and Classless
IP Addresses, which are a result of using various
subnet masks.
When we use IP Addresses with their default subnet
masks, e.g 192.168.0.10 is a Class C IP Address so
the default subnet mask would be 255.255.255.0, then
these are "Classful IP Addresses".
On the other hand, Classless
IP Addresses have their subnet mask modified in a
way so that there is a "Subnet
ID". This Subnet ID is
created by borrowing Bits from the
Host ID
portion.
The picture below shows us both examples:
I hope that you have understood the new concepts and
material on this page. Next we are going to talk
about subnet bits, learn how to calculate how many
bits certain subnet masks are and see the different
and most used subnet masks available.
If you think you might have not understood a few
sections throughout this page, I would suggest you
read it once more :)
|
