An administrator wants to create four subnetworks from the network address 192.168 150.0 24

Before we discuss how to implement it it is useful to understand why and when we need to do it and to do that we are first going to work through a simple analogy to illustrate the problem subnetting solves

Subnetting Analogy

As an analogy imagine a school and we need to split it into class rooms.

But why split it into class rooms? The answer is to stop classes interfering with one another.

Now each classroom has a desk with a computer and we have been tasked with creating a labeling system for the computers.

Now say we have 30 classrooms each with a maximum of 30 students and computers.

If we assign numbers to our classrooms and computers then we could have for example:

computer 11, classroom 24

We need two digits for the classroom which would allow for a maximum of 100 classrooms (0-99).

We need two digits for the computer which would allow for a maximum of 100 computers (0-99).

If we also say that classroom numbers 0 and 99 and computer numbers 0 and 99 were reserved and not allowed to be assigned then we now have a maximum of 98 classrooms and 98 computers which is enough for own requirements.

So lets create our label we could use the following scheme:

• computer 11, classroom 24
• 24-11
• 11-24
• 2411
• etc

There are many possible permutations we just need to pick one and tell every one about out labeling scheme.

Lets assume we go for 2411 where 24 is the classroom and 11 is the computer.

So now when we see the following 0223 we now know that this refers to classroom 2 and computer 23.

This we do easily in our heads once we know the labelling scheme.

We could even make it easier for ourselves by creating a paper mask that we put over the label that would reveal the classroom.

IP Addresses and Subnetting

Just like in our classroom example an IP address is split into two components a network component and a node component.

So the address 10.0.2.1 is split into Network plus Node.

So is the network number  10, or 10.2 or 10.0.2 ?

In early IPv4 networks address classes were used to identify the number of bytes allocated to the network component.

The main classes were class A,B,C. With the allocation as follows:

Class A    network,node,node,node
Class B    network,network,node,node
Class C    network,network,network,node

To determine the class you needed to examine the most significant byte (far left).

• 0-127 Class A
• 128-191 Class B
• 192-ccc Class C

IP Subnetting- Problems with Large Networks

All modern networks use the Ethernet data link protocol.

Ethernet uses a shared media and is negatively effected when a large number of nodes are connected to the same media.

This is just the same as having too many kids in the same classroom.

You can equate a network and node address to our classroom and desk number

What would happen if you had a classroom with 100’s of desks i.e. hundreds of pupils?

So even though a Class A address can accommodate thousands of nodes it it totally impractical to put this many nodes on a single network.

The solution to the problem was to split the network into small networks called sub networks or subnets.

Take for example a class A address which uses 1 byte for the network ID and 3 bytes for the Node ID. Written

Net.Node.Node.Node

It is important to understand that the network part of the address is only used for routing IP packets on the public internet.

Once the packet enters the private network then the Node address is used and the public Network address is not used.

Now a network administrator can interpret the node address any way they want, and so it is possible to split the node address into subnet and Node. So we could have

Net.Subnet.Node.Node or Net.Subnet.Subnet.Node.

The technique used to create subnets is to use a Mask.

The mask effectively hides the Node component and leaves the network and sub network components.

If the IP address was printed on paper we could hide the last byte by placing a paper mask over the number with three holes.

Note: if your binary is a little rusty see the binary numbers tutorial

To do this on a computer we use a number which we then logically AND with with IP address.

Here is the logic table for AND

1 and 1 =1 1 and 0 =0 0 and 1 =0

0 and 0 =0