1. Internet Protocol V4 Addressing & Subnetting
Written by Bill Reed 22/02/04

2. We will Look at the structure of an IP V4 address.
Examine the Binary principles behind an IP V4 address.
Identify the historic classes of IP V4 addresses.
Divide an IP V4 address into subnets.

3. IP V4 Address Structure Notation
IP V4 Addresses are represented in a way known as DOTTED DECIMAL NOTATION
An example of this could be
Each Decimal number represents a BINARY BYTE and therefore is limited to a value between 0 and 255

4. IP V4 Address Structure Hierarchical
IP V4 Addresses are hierarchical in nature
Think of a postal address.
The first Decimal number could represent a country.
The second Decimal number could represent county.
The third Decimal number could represent a post code denoting a town, an area of that town and a street.
The fourth Decimal number could represent a house number on that street.
An advantage of hierarchical addressing is that additions can easily be made, for instance we could build another few houses in our street and give them numbers without effecting upper layers of our addressing structure. We call this scalability.

5. BINARY PRINCIPLES How Binary values work
BINARY BIT VALUES
Eight BITS = 1 BYTE or OCTET
If all eight values in a BYTE are added together they come to a value of 255

6. How Binary values work
BINARY BIT VALUES
128

7. How Binary values work
BINARY BIT VALUES
128
192

8. How Binary values work
BINARY BIT VALUES
128
192
224

9. How Binary values work
BINARY BIT VALUES
128
192
224
240

10. How Binary values work
BINARY BIT VALUES
128
192
224
240
248

11. How Binary values work
BINARY BIT VALUES
128
192
224
240
248
252

12. How Binary values work
BINARY BIT VALUES
128
192
224
240
248
252
254

13. How Binary values work
BINARY BIT VALUES
128
192
224
240
248
252
254
255

14. DOTTED DECIMAL NOTATION
RECAP
IP V4 addressing is represented using
DOTTED DECIMAL NOTATION
Each DECIMAL number represents a BINARY BYTE
A BINARY BYTE can represent a value between 0 and 255
IP V4 addressing is hierarchical and therefore scalable

15. The Historical IP V4 Address Classes
Class A range = – BIN 0???????
0 = = (16,777,214 available addresses)
Class B range = – BIN 10??????
128 = = (65,534 available addresses)
Class C range = – BIN 110?????
192 = = (254 available addresses)

16. Dividing an IP V4 address Subnetting
IP V4 addresses have things called default subnet masks
Subnet masks tell some networking devices
how we want to divide our address space
A class A default subnet mask looks like
A class B default subnet mask looks like
A class C default subnet mask looks like
We subnet to gain additional discrete network addresses from a single address space.
Discrete network addresses are advantageous for security and organisational reasons.

17. Dividing an IP V4 address Subnetting
CASE STUDY
Dividing an IP V4 address Subnetting
= ONE NETWORK with 254 available addresses
Our client has bought this class C address space from her ISP.
Our client has a building with 5 floors.
Our client needs a different network address space on each floor.
HOW DO WE ACHIEVE THIS AND SATISFY OUR CLIENT?

18. Dividing an IP V4 address Subnetting
CASE STUDY
Dividing an IP V4 address Subnetting
Firstly we identify the address class.
Next we identify the default subnet mask.
Then we get out our DIY subnet calculator.

19. CASE STUDY
DIY subnet calculator
We inform the network devices that we want to divide up
Our address space by specifying a subnet mask that borrows BITS from the host portion of the address space.
128
192
224
240
248
252
254
255
BINARY values
Accumulated BIT values

20. CASE STUDY
DIY subnet calculator
Remember
We need
5 subnets
IP V4 address
Subnet mask
128
192
224
240
248
252
254
255
When we subnet using the no math method
we start from the right hand side.
We move left along the numbers until we
See a number larger than the amount
of subnets that we need (8), this will be the number
Of subnets that we create.
Next we count how many BITS are to the right
Of this number (in this case there are 3 BITS)
This is the number of BITS we must borrow.

21. CASE STUDY
DIY subnet calculator
IP V4 address
Subnet mask
128
192
224
240
248
252
254
255
So far we have established that we need to
Borrow 3 BITS from the host portion of the
Address.
Now we need to specify a subnet mask that tells the network devices of our intentions.
We achieve this by changing the DECIMAL value in the host portion of our subnet mask to a value that represents 3 BINARY BITS added together.

22. CASE STUDY
DIY subnet calculator
IP V4 address
Subnet mask
New subnet mask
128
192
224
240
248
252
254
255
So far we have specified a subnet mask to create 8 subnets
We borrowed 3 BITS from the RHS to achieve this
Now we must find out how many addresses are available inside each of those subnets.
We do this by counting from the LHS the number of BITS borrowed (3) and taking 2 away from this number.
32 – 2 = 30

23. DIY subnet calculator That’s it we are done!
CASE STUDY
DIY subnet calculator
Remember 32 was our spacing for the subnets
Our eight subnet addresses are:
That’s it we are done!
Now you know how to subnet!
An interesting aside is that the last octet of the last subnet address, matches the last octet of the subnet mask. (this will always be the case no matter what the subnet mask)