IPv4 8 VLSM 1 transcript

In order to understand VLSM, you need to understand FLSM first. Okay. So, uh before you start with the VLSM, make sure that you have watched FLSM. VLSM is variable length subnet mask that will be mostly used but it is little bit complex. So, what is it? VLSM is a subnetting techniques that allows a network to be divided. Look at here. That allows a network to be divided into subnets of different sizes. So that's the key.

So you have one network and you will be dividing the subnet. You'll be dividing the network into multiple networks but of different sizes based on the number of hosts required based on the number of hosts required. So in in in one case maybe you need eight hosts in another case you need 50 hosts. So based on the number of hosts you are going to divide the network. Instead of giving every subnet the same number of IP address VLSM allows the network administrator to allocate large subnets to networks that may hosts and smaller subnets to networks that need fewer hosts.

Okay. Okay. networks that need many hosts and smaller subnets. So you can have large subnets to a network that need many hosts and you can also accommodate like a fewer hosts in a small subnets. So your subnets will be of different sizes. This helps use IP address more efficiently and reduce wastage. Okay. Now if you go to the FLSM just imagine in case of FLSM if you have different variations of the requirement then FLSM is not uh efficient because you get the same block of IP address for each subnet.

Now let's have a look at the difference between FLSM and VLSM. Okay, all subnets have the same subnet mask. So if it is /24, everyone will have SL24. In VLSM, subnets can have different subnet mask. It can have SL24, it can have SL25, it can have SL26. Okay, in the same network, not in the same network like in in the same organization each subnet has equal number of host. So when you have the same cider value that means you have the equal number of hosts and when you have different cider values that means you will have different sizes and you will have different uh number of hosts.

Inefficient because many addresses may be unused many usable IP addresses may be unused. Efficient because addresses are allocated based on need. So it is efficient because it will be based on needs. Less flexible because everything is fixed. Okay. more flexible. Uh you got a subnet, you can again break it into smaller subnets. Again you can break it into smaller subnets, simpler to design, easy to do. It's little bit complex.

It's little bit complex. So for example um if you divide u /24 like if you uh if you want to divide /24 you can make it four four equal subnets with /26 and a / 24 network divided into 25 26 26 subnets based on the host requirements based on the host requirements. Now uh let's begin with this one. Okay. So if you look at here very carefully okay if you look at here let's say this is / 24. Now if you divide like if you want to divide this into half it will become like if you do slash25 then you get two.

So this is /25 this is also slash25. Now this is /26. Now look at here when you divide when you get to slash26 /25 into div will be divided into halves. Okay / 26 / each of them is slash 26. Now when you again make SL26/27 it gets divided into halves sl 27 /27 /27 so all of them gets divided /27 respectively all of them like in /28 again it gets divided into half and slash29 okay now if you look at here very carefully when it was slash4 I'm just saying the total number of IP address.

I'm not saying about the usable IP addresses. That means this includes both the network address and the broadcast address. Okay. So if you look at here when it was / 24 it was 0 to 255. All the numbers were in the same in the same place like in the same room. Okay? Okay. Or you can see in the same pool when it was /25 what did happen? The the room got divided into halves. So um if you look at here so uh when we divide it into halves it will be like because the number of addresses like all the numbers are the same you cannot increase.

So you cannot go zero below zero like - 1 -2. You cannot go above 25. So when you divide it into halves you get 0 to 127 then 128 to 255. This is for when you make it /25. When you make it this is also SL25. This is also SL25. Now when you divide /25 again when you make it SL26 it gets divided into halves. So again it gets divided into halves. So previously you had 0 to 127. Now it will be 0 to 64 63 64 to 127. If you see the numbers are the same.

There are 64 numbers. 64 numbers 64 64. Okay. Now if you look at here you got like for example four equal rooms with equal number of IP addresses or four equal four subnets with equal number of IP addresses. So this can be an example of FLSM. Okay. Like you are using uh four of them. Okay. But however like it the VLSM is beyond FLSM. Okay. Now if you you can also divide this and if you look at here I actually ran out of spaces to write them properly.

So I wrote this is the it starts with 128 and it ends in 143. It starts with 144 ends with 159 starts with 160 ends with 175. Okay. Uh again if you do this is slash 29. Again if you do /30 it will get divided more like it is going more and more. Okay. Now look at here very carefully. Let us take this one. So if you come here let us take this one. So if it was /24 you had 256 total IP addresses but usable IP addresses was 254.

Why? Because you need to subtract two. One is the network address another is the broadcast address. When it was /26, you had 62 usable IP addresses. When it is SL27, you have 30 usable IP addresses. When it is /28, you have 14 usable IP addresses. Now, if you look at here very carefully, this is SL28. So, if you look at here, we have 0 to 15. That means total 16 IP addresses, 14 is usable, then 16 to 31, then 32. So, if you look at here, you have 14 usable IP addresses.

Now what we going to do here is Okay. So I have this table and I have this one. Okay. Let me take a requirements. Let me take a requirements. Now if you look at here very carefully, I have given three requirements. What are the three requirements that I have given? There is IT department, there is accounting department and there is marketing department. Uh IT department has eight people, 50 people, 60 people. So what is that?

Uh I need 60 IP addresses for marketing department. I need 50 IP addresses for the accounting department and I need eight IP addresses for the IT department. Now if I come here, so if you let me let me write down the requirements. 8. So if you remember for uh let me have a look at it again. So it accounting 50 marketing 60. IT8 accounting 50 marketing is 60. Now look at here very carefully. If I need eight IP addresses.

So if you look at here um here I have six usable IP address. If it is /29 then I have six usable IP addresses. So it doesn't solve my problem. But if I take /28 okay /28 then it solves my problem. Okay it solves my problem. Okay. Now look at here very carefully. So I can take one one from/28 because it produces. So let me take just this one for the I will take this one for the IT department. Now for accounting department I need 50 IP addresses.

Here I have only 14 usable IP address 50. No it cannot solve the problem for 50. Here 30 it cannot solve as well. So um the one that can solve the problem I can see nearest is 62 which is which is like um you need something which is greater than or equals to 50. Okay so that has solved the problem. So I can take what is the value it's/26. So I can take one from slash 26. Okay let me just take one from slash 26. Okay.

Now I will take this portion of this one because I want to show you something. Okay. Now um then we have another one uh marketing is 60. So I know that this 60 can solve the problem. So let me take this portion for marketing. Okay. So this is for IT, this is for accounting, this is for marketing. Now is this right? How can you understand this is right or wrong? Okay. But I can see like it fits. Okay. Like uh what's the difference between U FLSM and VLSM?

So if I had to take FLSM for example I needed at least three networks. With FLSM I needed at least three networks. So let me do the FLSM here with um with green color. Okay for FLSM I needed at least three networks. Do I have three networks here? Nope. I have here three networks. Okay. So I have here three networks. I have four networks. So I can take this. According to FLSM all the networks will have to be of same size.

Okay. So all of them are of safe same size. Okay. And you have 62 usable IP address. If you come here it's SL26. You have 62. Now look at here. If I bring the IT department here for IT department I need eight IP addresses. But how many IP addresses I have? I have 62. What will be the wastage? So if you see here almost 70% IP addresses are wast wasted. Okay. So this is not efficient. Yeah this is it is good. It works.

It works u but this is not efficient. Okay. So that is why this is more efficient for IT department. So FLSM is not going to work here. Now let's come here. Why? Let's see. Okay, you you remember this block. Let's come here. If you come to this block here, just take the IP address. So, it will be let's say we take a class C IP address. Okay, 192 168. Um, so it will be this is the network address and this is the broadcast address.

So for accounting department 192 168 dot because we are doing /242 2526 it has we we do it uh uh uh let's say class C okay we can do with class B we can do with class A as well doesn't matter 192 168 dot 0 dot let's say the first one is sorry I forgot here 128 and last one There is a mistake here. Okay. There is a mistake. So it cannot be um it cannot be 190. Okay. So it will not be 190 here. So if you look at here if it is SL26 let us look at the table for slash26 for/26 it will be 191 1 it will be 191 and it will be 192.

It will be 192. Okay. Uh there was a mistake here. So it will be 191 it will be 191 and it will be 192. Okay. The formula is the broadcast address will be always an odd number. Okay. If you see the broadcast address is an even number that means there is a mistake somewhere. Okay. The broadcast address has always to be an odd number. So this is 191. This is 192. Okay. Okay. Based on this, if I just write here 190 to 168 dot just uh 128 to 191 and 192 168.0 191.

Okay. For marketing department it is 192.168.0 0 dot 192 to 192 168 dot 0.255. Okay. Now come for the IT department. See what we have here. 1 2 3 4 number five. Okay. 1 2 3 4 number five. If I see that. So 192 168 dot 0.1 176 192 168.0 Okay. Now look at here very carefully. What's the problem? The problem is this accounting department and marketing department they have overlapping IP addresses. They have overlapping IP addresses.

Okay. Because this is the one that you can only get if you divide these if you divide this if you divide this field. Okay, if you divide this field only then you can produce this one. But you have already used this field. So how can you divide? If you are using a room, you cannot divide it. If you if you already have a house, you cannot build another house on the plot. If you have an empty plot, then you can divide it and you can build you can divide it into two halves and you can build two houses.

So you have already used this field. So you cannot divide it anymore. If you have to divide something, you need to divide something which you haven't used it. Okay? So this cannot happen. Now the question is you might say this is isn't it SL26? Yeah, this is SL26. This is also SL26 and this is SL28. It doesn't matter because you get SL28 by dividing by from SL26. So you already use SL26. So you have you cannot divide it anymore.

Okay? So you cannot use this part of the SL26. However, this part of the SL26 is empty. This part of the SL26 is empty. So, you can easily use them. So, it is not it is easy but it is not that easy as well. Okay. Uh you need to understand. Okay. It is also complex. So, what we can do here is actually let's remove this part here and let's take this one. Sorry. Or let's take this one. Okay. So if I take that one then it will be 192 168.0.0 and 192 168.0 dot I think it will be 15 here.

It will be 15. I think this is the one that we taking. Yeah, it will be 15. 15 then slash 28. Okay. Now if you look is there is there any overlaps? There is no overlaps. Okay. Now your question could be we still have wastage where we have wastage. For example, we have here 62 IP addresses but accounting department needs only 50 IP address. There will be a little bit of wastage. Okay. And in fact if you think practically that is also not wastage.

The reason is what if one or two people joins a accounting department in one or two months. What if uh 10 people joins in one in five years. Okay. So you don't have to redesign the subnet again. So is not an it's yeah apparently it is an wast wastage but uh overall okay if you think overall that is what we can go okay but it is better it is better than using let's say for example if I use this one I will have 254 right now if I come here for the same thing so if you look at here for the same thing if I come here you see here for the accounting uh for the IT department for the accounting department I used 1926 16850.0/24.

So you this is your network address 50.0 and this is your broadcast address 50.255. So what's happening? You are having 254 usable IP addresses. That means you are wasting more than 200 IP addresses is better than that. Okay. So this is like a this is like a FLSM. Okay. This is like FLSM you can say. Okay. But um this is the one that we did here is the true realism. Now the question is can we take anything from here?

Uh of course you can take any field but you need if you take it sequentially then it is it looks more good. Okay. And it is it is highly likely you'll not make a mistake. But if you pick the fields randomly from here and there then highly likely as I said there could be overlapping IP addresses and you could get into trouble. Okay. So this is an easy realism. Okay. Uh this is an easy realism. They are different networks.

They are different networks. They cannot communicate with each other. Okay. Even though if you look at here they look the same like 1916 0 19168 but they are in different networks. Why they are in different networks? Because if you look at the subnet mask, even the subnet masks are same here, but they are still the different networks. Okay. Um if you do a little bit of practice, you will understand the based on the subnet mask, their IP addresses got changed and they're in different networks.

And here the subnet mask is different. As soon as I see the subnet mask is different, I know that straight away this is in a different network. Now um if you um okay this this table is fixed okay now I think it's not easy to memorize this one but it's easy I I memorized so it's easy it's not that hard the only problem is during the exam you will be given with a lot of numbers let's say uh you'll be given with seven or eight okay and with we are seeing something here like which is less than 250 or something which we can solve with class C.

However, you will be given uh a department with 500 IP addresses, a department with 1,000 IP addresses, which cannot be solved with class C. Okay? And it will get harder. So therefore, we should have something. Is there a way we can do it? The best way, the more you practice, the more faster it will become and um you will get used with it. Okay, this is this is uh the illustration I used only for understanding but this is not uh this is not practical this is not feasible to do during the exam drawing and writing those things so you need to have some mathematical calculations and formula to do this one okay let's say we want to solve this one okay uh this is the one that we have been given and I have some formulas here what are the formulas when someone gives you something let's say someone has given you okay like for example here we got here um 8 50 60 okay so they will give you randomly the first thing is when solving a VLSM problem the first step is to list all the required subnets from the largest number of hosts to the smallest so you take the requirements and you arrange them from the largest to the smallest okay this order is important because larger network requires more addresses and they must be allocated first to avoid wasting address.

Now if you come here and if you look at here if you started taking like if you if you already take the accounting and the marketing from /26 you know that you cannot take this field for it. Why? Because you know that you have already taken this. So these things are already dead. So these things are already dead because this part is already gone. Okay. But had you started with it from here okay for example. Okay. Then you started accounting.

Now you need to remove the it from here. It would create a mess. Okay? So never start with the smallest one. Always start with the largest one. Now uh start allocating from the largest requirements. So always try to solve the problem with the from the largest host requirements. Okay? Like for example 120 50 and gradually move towards the smallest. Now look at here. You need to you need to follow the sequence. Okay, that means you need to go sequentially.

You cannot just pick randomly from here and there like I do this one then I do this one. No, it doesn't work. Okay. Uh use IP address uh efficient IP address space. Okay, always check that one. Make it efficient. Um then no overlapping between the subnets. No overlapping between the subnets. This is very important. You cannot overlap between the subnets. A smaller networks can fit into the remaining address space. Okay?

You cannot have overlaps. Okay? the one that I showed you in the first place where we had an overlap with the IT departments and with the accounting departments. You cannot have overlaps because every device has to be identified with an unique IP address. Now identify the appropriate class of network address. Now when you got the requirements for example if someone says I need 500 hosts with class C the max usable host you can get is 254.

So you cannot get 500 with class C. So that is why you need to use class B. So make sure that when you select the class in the first place, you select the appropriate one. This is only possible if you start with the highest number. Okay? If you start with the with the highest number. Okay? Uh because um without the high highest number is not going to happen. Okay? So I think you can write here highest number. Okay. Whatever.

Now for class B it it comes like this default. Okay. Now after selecting the network address move the cider value to the right. Now look at here very carefully. We know /24 is the cider /25 is the cider / 16. These are the cider. You can move the cider value to the right. So if you start with class B so let's say this is the default that you have for class B and for 500 for 500 hosts how many IP addresses do you need for 500 hosts how many IP addresses do you need now look at here very carefully if I do 2 to the power 8 I will get sorry if I do 2 to the^ 9 then I will get IP addresses and if I subtract two then I will get usable IP address usable IP address okay addresses whatever what is this 2 ^ 9 is the number of host bits is the number of host bits so I need only nine host bits so if you take this one how many host bits do you have here Okay, you have here total 32 bits minus 16 network bits.

You have 16 host bits. But how many host bits do I need? I need only nine. So I can donate I can donate or you can say borrow. Okay, seven to the network bits only. If you give seven to the network bits then you will have 9 and the cider 16 + 7 you will find is /23 okay so what is going to happen the / 16 will become /23 okay because I don't need this I have total 16 host bits I don't need 16 host bits I need only nine host bits so what I need to do I need I I will give away seven bits, seven host bits to the network bits.

Okay. And in this way my cider my network bits will increase from 16 to 23 because 16 + 7 is 23. Okay. Now look at here very carefully. You you you might have to watch this part again and again because this is a little bit complex. Determine how many bits are borrowed and create a subnet table. So if you look at here the new subnet is /23. So we are borrowing seven bits from the uh from the host bits. Seven bits can create a subnet combination.

So with the seven bits you can create 1 2 3 4 5 6 7. There you don't have eight bits. So you have 1 2 3 4 5 6 7 just seven bits. Okay. With the seven bits you can create 2 to the^ 7 which is 128 combination. Okay. So you can create 128 subnets and each of them is unique. Okay. So the binary values represents the possible subnet numbers created by borrowing bits. Each combination responds to a different subnet which is within the original network.

So you have divided. So previously you had a big network. Now you have divided this network into 128 networks. Okay. Okay. Like for example, if you look at here, we had a big one and we divided into two. Okay. So if you have SL16, it was a big one and when you do SL28, sorry, SL23. So you have one big network with SL6 and when you did SL23, actually we get 128 networks. Okay, the you will get 128 networks and these 128 networks can be used for example 128 departments.

Okay, now uh let us come here and let us see what we have here. So if you look at here uh let's say we took 192 168 1.0 okay and um we need for example uh we'll start with the highest number. So if we start with the highest number um then how many uh what will be the one that we have 60 50 and 8. So we'll start with 60. So how many how many host bits do you need for 60? So if you do 2 ^ 6 6 - 2 you will see you will see you get uh if it is 2 ^ 6 it is uh 64 and minus2 is you have 62 IP addresses.

Okay. So if you look at here so I need only six host bits. So if I take this number, so I took 192. So first of all 192 168 1.0/24. How many host bits I have here? There are eight host bits. But how many host bits do I need? 192 168.1.0. I'll make it / 26 because I need only six host bits. Is that clear? I need only six host bits. So when we are doing when we are making from eight host bits to six host bits, we will borrow two host bits and we will give it to the network bits.

And if we give it to the network bits, it will be /26. When you give it to the network bits, when you give two host bits to the network bits, if you look here, what happened? Two bits, you can create eight combinations. you can create 128 combinations. So with seven bits you can create 128 combinations. So with two bits you can create with two bits you can create 0 0 1 1 0 1 1 combinations. The formula is 2 to the power number of bits.

Okay? So four combinations you can create four combinations. So if you look at here, I'm creating these four combinations. When I'm doing from SL24 to SL26, I get these four combinations. This is table one. Let's say this is table one. There are four subnets. There are four subnets and each of them can give you 62 usable IP. Now take the first one from the table from table one. First one from table one. Take the first one from table one.

Then what you see here according to the what is the network address and what is the broadcast address? According to the formula network address all host bits zeros all host bits zeros and according to the broadcast address all host bits once. Sorry make all host bits zero and make all host bits once. So if you look at here 190 to 168 one dot these two bits are there you cannot change it okay because when you are donating when you are borrowing this when you are borrowing these two bits it becomes fixed.

Okay so you are taking the first one you are taking the first one from the table. Leave the three in the table. So when you're taking it is /26. So you cannot touch the network portion. So it will be again 128 192 168.100 0. Now make all the host bits zeros. Make all the host bits on. So if you look at here, if you add this is the network address and this is the broadcast address. Okay. So this will solve the problem for 60 IPs.

Now allocate subnet based on the host requirements. So according to here you have got according to table one you have got four subnets each of them 64 just take one subnets and uh put it for a particular department. Okay do not modify bits that have already been allocated. Okay once you allocate a subnet from a table the borrowed subnet bits the borrowed subnet me must remain fixed. So you cannot modify it. So if you look at here this is the network portion for /23 this was the original network portion these are the subnet bits that I get I borrowed and they become the part of the network portion and you cannot change them okay if I'm making from /16 to /23 you cannot change them the only portion you can change is the host portion this is the only portion that is flexible you can only change here now according to uh this formula.

Okay, according to this formula, if you come here again, so I use the first one from the table and I cannot use it anymore because this is done. This is already used for a department. We can take the second one from the table. So the first one is already done with the 60 people. We'll take the second one from the table. So which is 01. So if you look at here 1.01, 01. What is the formula again? Uh, for the second one, wait, wait, wait, wait, wait, wait, I made a mistake.

Sorry. Let's have a look. Okay, this is the second one. Sorry. This is the second one. Okay. Uh, I I went here. Forget about this part. So this is the second one. Take the second one. Again look at here. The first one from the table is this one. The second one is 192 161 01 01. This is the second part. So let's do here 192.168.1. So once you borrow the bits, you cannot change them anymore. Okay? You cannot change them anymore.

You cannot modify them anymore. You need to leave them as it is. the network address and the broadcast address. So if you look at here if you add if you add those 128 + 64 + 32 if you add them you will get 64 is the network address and you will get here is 127 is the broadcast address. And look at here how beautiful it is. For the first one the network address is 0 to 63. That means all the IP addresses are between 0 to 63.

For the second one all the IP addresses from 64 to 127. So you used the second subnet from the table. So if you look at here, the second one is also done. Okay. Now you are left with two of them. And take one of them from here. Okay. I will take this one, the third one from the table. Let's take the third one from the table. 192 168 1.10. Now the issue with the third one is I required here 60. I required here 50. Okay?

But I required here eight. But how many IP addresses do I have here is 62. Why? Because it is slash 26. I saw the opportunity. If you come to the table here, if you look at here, if I make it /28, I will have 14 IP addresses. Okay, 14 usable IP addresses. So why don't I take it SL28? Isn't it that is what is the VSSM? So we will make here / 26. So if I draw here the table you already have 192 168.110 which is /26. Rest of them is there.

Now I'm writing this one into decimal format and I'm writing this into binary format. This is because of understanding otherwise it will be it will take a lot of space for me. Okay, a lot of space for me. Now if I do here 192 168 1 0 and if I again want to borrow two bits then it will be 0 0 I will have to create another table 192 168 dot 1 dot 1.01 01 192 168 dot 1 dot then 1 0 192 168 dot 1.11 then 1 one so again I divided this one into four four parts okay now it will become slash 28 it will become slash28 now take one from this table one from this table okay so if I take the first one from the table.

So if you look at here, these are the part of the subnet bits. That means I gave it to the network and I cannot change it. So you you keep this one fixed 192 168.1 0 0 and according to the network address all host bits zeros according to the broadcast address all host bits one. So it is 128 to 143. So previously we had from 64 to 127. Now if you look at here that that was for 50 host. Now you have 128 to 143. It is SL obviously it is SL28.

It was SL26. However there was no there is no overlapping. There is no overlapping. So you can use this one. A recap. Okay. Arrange them in descending order. Okay. Always start with the largest one like the highest number. Okay. or largest host requirements and move gradually to the smallest requirements. Identify which class is required and that should be done based on the first problem that you start solving based on the first subnet that you want to solve the problem.

Okay, based on the first requirements and you you will always be using class B if you are using that one. Number four, the cider value can move to the right only. So you cannot move it to the left. Okay. So you can borrow you can borrow bits from the host portions only and your cider will move slash will move from left to right. Now when you are moving okay when you are moving from left to right okay you will be creating subnets with the number of borrowed bits each of them is a subnet and you should write them okay like if you borrow seven bits so it creates 128.

You don't need 128 networks. You know that. But you know that you create 128 subnets and you each subnet is unique and they can be used for solving each of the problems. Okay. So if you took one uh assign one subnet to the assign one subnet for each host requirements um then take the next available subnet. Okay. then take the next available subnet. Like this subnet if you see it can be broken into another other subnets as well.

Let's say you took a subnet with SL26 and it can be broken into four other subnets with SL28. Yes, you are you can you can break that subnet. Okay. Like for example uh if you look at here in this diagram we we took these two subnets and after that we broke again these subnets here. Again we broke it here in order to get our requirements. But the one that you used, you cannot break it. The one that you have used, you cannot break it.

And the borrowed subnet bits must once you borrow the subnets. Okay? Like for example, if you look at here, you create here 128 subnets and this portion become fixed. These portions become fixed. You cannot change it anymore. Okay? Look, this portion is already changed here. And again, if you look at here, this portion is already changed here. You you have already changed it. There is nothing to change. If you change it to something else actually you will create a match with something else and that will be the same.

Okay. Okay. Uh subnet into different sizes. Okay. So you'll have different sizes. It will be efficient and it will be less wastage. Okay. Uh so you try uh you try doing uh this one. This is from class C but obviously you may get from class B and class A as well. But always practice a few of them with class C so that it becomes much more easier for you.