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.
Subnet Mask
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?

Credits

[curesulunga]

thank you for posting these for free! 2 years later and my class is still using this video :)

[shabarishnarayan]

Hi, your videos are awesome... They way of demonstration is really great and understanding... Please publish more videos like this. Great work.. Am was really waited a lot for your videos... Thanks coming back...

[StriderRyu]

Thanks for this....Reading books about this can be a bit dull sometimes.

[k.chriscaldwell]

An easier way to convert binary to decimal and back, and with experience, do it in one's head, is to:

1) Take b10111010 and break in to two four bit parts, _nibbles._ Like so: b1011 b1010.

2) The right most place in the left nibble is the decimal 16 place. Convert the left most nibble to its decimal value, 11. Multiply that value, 11, by 16 to get 176.

3) Compute the right most nibble’s decimal value, 10.

4) Add the two nibble decimal values attained above to get 186. Done.

Going the other way, recognize that 186 is less than 256, and so divide 16 into 186 to get 11 remainder 10. Convert the quotient of 11 to its binary value, your left nibble of b1011. Convert the remainder of 10 to its binary value, your right nibble of b1010. Slide the two together, done.

Learning one's times-table at 16 helps.

[whatskills]

10:30 CIDR 20 has 4094 addresses not 4064.

[SunnahFollower]

Keep your explanation simple and precise, don’t use ambiguous vocabulary.

You’re just going to confuse people about simple concepts.

[kavinduvindikasomadasa]

not good for beginners...just saying things he

[sangamshrestha]

I am not getting it... Hard to understand.. You are not teaching but you are giving answers to the teachers...