1. Introduction to Networking (Yarnfield)
Classful subnetting

2. Objective To cover the subnetting process
Be able to subnet from a variety of information

3. Subnetting
The process of dividing a network up into subnets and to assign each subnet a valid network IP address, and the hosts on that subnet valid IP addresses
Subnet 2
Subnet 1
Subnet 3

4. We need to find out How many subnets does the subnet mask provide?
How many valid hosts per subnet are available?
What are the valid subnets?
What is the broadcast address of each subnet?
What are the valid hosts in each subnet?

5. We need An IP address to subnet And
The number of subnets we wish to make Or
The subnet mask
The number of borrowed bits

6. Example

7. Example IP address: 192.5.5.35 Subnet mask: 255.255.255.224 Work out
Network ID Class
Major network address
Major broadcast address
First host of the major network
Last host of the major network
Class C
The network ID class is determined by observing the FIRST octet. Bear in mind the following:
= Class A
= Class B
= Class C
The major broadcast address is the very last address in the network.
The address of the first host on the network is always immediately AFTER the network address.
The address of the last host on the network is always immediately BEFORE the network broadcast address.

8. Example IP address: 192.5.5.35 Subnet mask: 255.255.255.224
Now we need to work out the subnet information
Number of subnet bits
Number of host bits
Subnet address for the given host IP address
Range of host addresses for this subnet
Broadcast address for this subnet
3 subnet bits
5 host bits
To determine the number of subnet bits you must convert the last octet of the subnet mask to its binary equivalent and count the number of one’s (or ON bits) in binary is , therefore there are three one’s, meaning we have used three bits to create subnets with.
To determine the number of host bits you must convert the last octet of the subnet mask to its binary equivalent and count the number of zeroes (or OFF bits) in binary is , therefore there are five zeroes, meaning we have five bits to create host addresses with.
To determine the subnet address for the given IP address we first subtract the value of the last octet from In our example =32. This is called the block value. To determine the other subnets in the scheme we add to block value to the PRECEEDING subnet address. Therefore our subnet addresses will be:
0+32=32
32+32=64
64+32=96
96+32=128
128+32=160
160+32=192
*Note* that we start at 0; this indicates the major network address. Also, we stop at This is because if we continue =224 which is our subnet address. We cannot use this as one of our sub networks.
Once we have worked out our subnets we simply observe where our given IP address falls within the scheme. Our example is This is greater than 32, and less than 64, so the subnet address for our given IP address is
The range of host addresses for the subnet can be determined once you have completed the above step. The range always starts immediately after the subnet address, and ends immediately before the broadcast address. In our example, is the subnet address, so our beginning of range is The end of range will be as is the broadcast address, and is the next subnet address.
The broadcast address is the address immediately before the next subnet. If our example subnet is and the block size is 32, the next subnet will be The address immediately before this subnet address is therefore , which will be used as the broadcast address for the subnet. –

9. Example IP address: 192.5.5.35 Subnet mask: 255.255.255.224
Now we need to find other subnet information
Total number of useable subnets
Number of valid host IP addresses per subnet
Convert given subnet mask to binary
Convert given host IP address to binary
Subnetwork address with slash (/) mask for the assigned subnet 6 30
To determine the total number of useable subnets we use the formula 2n-2 where n refers to the number of borrowed bits (or put another way, the number of bits that are turned on, i.e. 1).
In our example we have borrowed three bits, so 23-2=6. We have to take 2 away because this refers to the major network address and the subnet mask, which we can’t use.
To determine the number of valid host IP addresses we use the formula 2n-2. This time n refers to the bits we have not borrowed (or put another way, the number of bits that are turned off, i.e. 0)
In our example we borrowed 3 bits, therefore we have 5 bits left. So 25-2=30. Once again we have to take 2 away from our answer because this refers to the subnet address and the broadcast address, which we cannot use.
To convert the given subnet mask to binary and the given host IP address to binary you simply perform a decimal to binary conversion.
The subnetwork address with a slash (/) is simply a way to represent and IP address and its subnet mask conveniently without having to write the subnet mask. The number following the slash is simply the number of bits that are turned on (1’s) in the subnet mask. In our example our subnet mask is which is in bianry and if you count the 1’s is 27 one’s. So the format would be / 27
/ 27

10. Exercise IP address 202.168.2.40 Subnet mask 255.255.255.240 Find out:
Network ID Class
Major network address
Major broadcast address
First host of the major network
Last host of the major network
Number of subnet bits
Number of host bits
Subnet address for the given host IP address
Range of host addresses for this subnet
Broadcast address for this subnet
Total number of useable subnets
Number of valid host IP addresses per subnet
Convert given subnet mask to binary
Convert given host IP address to binary
Subnetwork address with slash (/) mask for the assigned subnet

11. Questions...
... are there any?