 # Subnetting

Welcome to the ITFreeTraining video on subnetting. Understanding how to subnet is essential if you want to deploy and maintain IPv4 networks.

Binary to Decimal
0:10 – In a previous video, I looked at how to convert from decimal to binary and binary to decimal. This is an important skill to know when subnetting so I will do a quick review. If I want to convert the following binary number to decimal I would use the following table. Each binary position has a value based on a power of 2. To convert the binary number, it is just a matter of adding it to the table. Using this value will give us the last row of the table. If the value is a 1, the power of 2 value is used, otherwise the total is a zero. Once all the totals have been worked out, it just a matter of adding them up to get the value. Now let’s have a look at converting decimal to binary.

Decimal to Binary
0:57 – Let’s consider the same value. To convert it to binary, the powers of 2 are used again. This time the value that is to be converted, 186 is compared with the powers of 2. If it is higher, which it is, a 1 is recorded. Since the value was higher, the powers of 2 value which is 128 is subtracted from the value. The new value, 58, is compared to the powers of 2 value. It is lower so a zero is recorded. The process is repeated until all the values are calculated. This will give the decimal number in binary. If you understand this process, then you are all set to start looking into subnetting. If you do not feel confident in your decimal and binary skills, I would suggest looking at our previous video of converting decimals as it goes into this process in more detail.

1:47 – Shown here are some examples of subnet masks. First, you can see subnet masks always have 1s on the left hand side while there are zeros on the right hand side. Due to this, notice in very first column, the last decimal in the IP address. Since 1s and 0s must be contiguous, certain decimals will always be used. Notice too, that the number of subnets and hosts that are available depending on which subnet mask is used. As the number of 1 bits in the subnet mask decreases, more hosts are available per subnet, however fewer subnets are available. Notice also, that in the top two subnets, there are no subnets or hosts available. To understand more about subnets, let’s have a look at why this is the case.

Network Prefix & Host Identifier
2:37 – The subnet mask divides an IP Address into a network and host identifier. So what happens when you are given an IP address like this one where all the subnet mask bits are set to one? First the IP Address and the subnet mask are applied. The will allow the network prefix and host identifier to be found. It is no surprise in this case, that the host identifier will be 0.0.0.0 this is because no bits have been assigned to the host part. When a subnet mask that is all 1s is used, the network prefix will always be the same as the original IP address. So the question remains, when is this useful?

Any host ID that is all zero bits is considered to be the network address. The advantage of having such an address is that when this address is used, it is clear that only the network is being referred to and not a host on the network. In contrast, any host ID that is all 1 bits is considered to be the broadcast address. A broadcast address, when it is used, will transmit the same network transmission to all hosts on that network. So how does affect the number of hosts that can be on a network?

192.168.0.0/24 Breakdown