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

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.

IP V4 Address Structure Notation IP V4 Addresses are represented in a way known as DOTTED DECIMAL NOTATION An example of this could be 192.168.100.17 Each Decimal number represents a BINARY BYTE and therefore is limited to a value between 0 and 255

IP V4 Address Structure Hierarchical IP V4 Addresses are hierarchical in nature. 192.168.100.17 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.

BINARY PRINCIPLES How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 Eight BITS = 1 BYTE or OCTET If all eight values in a BYTE are added together they come to a value of 255

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128 192

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128 192 224

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128 192 224 240

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128 192 224 240 248

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128 192 224 240 248 252

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128 192 224 240 248 252 254

How Binary values work BINARY BIT VALUES 128 64 32 16 8 4 2 1 128 192 224 240 248 252 254 255

DOTTED DECIMAL NOTATION RECAP 192.168.100.17 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

The Historical IP V4 Address Classes Class A range = 0.0.0.0 – 126.255.255.255 BIN 0??????? 0 = 00000000 - 126 = 01111110 (16,777,214 available addresses) Class B range = 128.0.0.0 – 191.255.255.255 BIN 10?????? 128 = 10000000 - 191 = 10111111 (65,534 available addresses) Class C range = 192.0.0.0 – 223.255.255.255 BIN 110????? 192 = 11000000 - 223 = 11011111 (254 available addresses)

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 255.0.0.0 A class B default subnet mask looks like 255.255.0.0 A class C default subnet mask looks like 255.255.255.0 We subnet to gain additional discrete network addresses from a single address space. Discrete network addresses are advantageous for security and organisational reasons.

Dividing an IP V4 address Subnetting CASE STUDY Dividing an IP V4 address Subnetting 192.168.100.17 = 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?

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.

CASE STUDY DIY subnet calculator 192.168.100.17 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 128 64 32 16 8 4 2 1

CASE STUDY DIY subnet calculator Remember We need 5 subnets 192.168.100.17 255.255.255.0 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. 128 64 32 16 8 4 2 1

CASE STUDY DIY subnet calculator 192.168.100.17 255.255.255.0 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. 128 64 32 16 8 4 2 1

CASE STUDY DIY subnet calculator 192.168.100.17 255.255.255.0 255.255.255.224 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 = 30 128 64 32 16 8 4 2 1

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: 192.168.100.0 192.168.100.32 192.168.100.64 192.168.100.96 192.168.100.128 192.168.100.160 192.168.100.192 192.168.100.224 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)