IPv4 7 FLSM transcript

Okay. Um, FLSM fixed length subnet mask. Let me just get this one here. Uh, it is a subnetting method. Every subnet has the same subnet mask. Now look at here very carefully what it says here. Every subnet has the same subnet mask. What do you mean by same subnet mask? So every subnet if they have SL24 then all the subnets will have SL24. If they are SL25, everyone will have SL25. Every subnet has the same number of hosts.

So if if one subnet has 14 host, the other subnet will also have 14. If one subnet has 30, the other subnet will have uh 30 uh 30 as well. like this number can vary but as I mentioned like they will have the same. It is opposite to VLSM. What is VLSM? Variable length subnet mask. So every subnet will have different lengths. Every subnet will have different lengths. Okay. Um so one can be SL24, other can be SL25, one can be SL26, other can be SL27.

Do we use FLSM? FLSM is used when all departments need equal number of hosts. when all departments needs equal number of hours. So for example, you will get a question something like this. If uh okay we need we have four departments four departments and each department needs let's say 30 IPs or 30 hosts okay or they have 30 computers PCs and the network design is fairly simple because everyone will have the same okay just make sure that you have a big room and you will divide it into four okay of equal size where 30 people can sit Okay.

Now look at here very carefully. Let's say this is the network address. So you will be given a network address or you pick a network address and you need to create four subnet. The formula is 2 ^ n greater than or equals 2. Okay. So you need to let's say for example if they say you need three subnets. Okay. So um if you need three subnets actually you can't get something like three subnets. So you can get either two subnets or you can get four subnets.

So three false in between them. So that is why I said greater than or equals. Okay. So if you look at here according to the question you need four subnets. N is the number of borrowed bits from the host bits. So if you look at here in this question the /24 means there are 24 network bits and there are eight host bits. So you are going to borrow two bits from the host bits. Okay. Now if you look at here 2 ^ 1 is greater than equals 4.

Borrowing one doesn't work. Okay. uh borrowing two 2 to the^ 2 is four greater than or equals okay four that works. So borrowing two bits it works borrowing one bit it doesn't work. Now look at here we will borrow two bits from the host bits. We'll borrow two bits from the host bits. Borrowing host bits means putting them into the network bits and you will need to form the networks. So when you borrow let's say for example this is your network bits portion 24 and this is your host bits 8.

When you borrow two bits from here this will become six and this will become 26 and this will extend whereas this will shrink. Okay, two bits from the host bits. Borrowing host bits means putting them into the network bits and you will need to form the network. You need to form the networks. Okay, so when you borrow two bits, you can form four networks. The formula is 2 to the^ two. We'll have a look at that. Borrowing bit will increase the size of the network bit and will decrease the size of the host bits.

In this example, you have eight host bits, 24 network bits. If you borrow two bits from the host bits, you will be able to produce four networks because the formula is 2 to the^ n and you will left with six host bits. So if you look at here uh then why do we borrow bits? Because borrowing bits uh create more network. Each bit you borrow doubles the network. Like for example, if you borrow one bits, you'll create two networks.

If you borrow two bits, four. The formula is very simple. The formula is 2 the power n okay 2 the power n is the formula based on that you can do that so when you borrow two bits in our example 19 to 168 1.0/24 will become 1968 1.0/26 0/26. Why? Because you borrowed two bits from the host portion. So that is why host portion got small. Host portion got small. So that is the one that is happening here. Now if you look at here very carefully 24 becomes/26.

The difference is in the fourth octed. Where the difference happened? The difference happened here in the fourth octed. The difference happened in the fourth octed. So / 24 became / 26. And with the two bits you can produce four networks. 0 0 1 1 0 1 1. It happens in the fourth octed. So if you look at here, it happens in the fourth octed. And which two bits happened? These two. These two. These two. These two. Here it happens.

Now look at here very carefully. It is now slash 26. It is now slash 26. Okay, it is it has become now slash 26. Before it was /24, it has now become /26. And that is why if you look at here, if you do the network address, all host bits zeros. So you only these six bits are zeros. it will be zero. So these two will be these two will be the network portion. Now these two will be in the network bits portion and these six is the host bits portion.

This is the host bits. Okay, this is the host bits. According to the formula, what is the network address? All host zeros. And what is the broadcast address? All host bits zones. Now you see look at here. If you make them zero 192 168.0 64 because why 64 0 1 64 1 0 there are four networks. Now look at here when you borrowed the bits you formed four networks. Now everything is fixed. You cannot make any changes anywhere.

Okay? There are four networks and you can make changes only in the six bits. Previously, you could make changes before you borrowed the bits. You could make changes with all the eight bits. Once you borrowed the bit, you need to create networks based on how many bits you borrow. Once you have done that, then you cannot do anything in these two bits because they are fixed. They are network bits. With the network bits, you cannot do anything.

So only you can make changes in the host bits and they will be slash there will be /26. Previously it was /24. So if you look at here I divided this /24 into four equal networks. So if you look at here 0 to 64 64 to 128 128 to 192 okay into four equal networks. Okay. So if you go um yeah so from 192 the end will be is not we are writing only the network address. Now look at here very carefully the subnet one this is the this is the network address and this is the broadcast address.

These are the usable host. For subnet two this is the network address. This is the broadcast address. In between you have the usable host. For subnet 3 this is the network address. This is the broadcast address. These are the usable. Okay. When you move from SL24 to SL26, we borrow two bits from the host portion and give them to the network portion. That's right. With two bits creating four different sub subnet combinations.

That's right. Is each subnet has got six host bits remaining. That's right. Each subnet has got four uh six host bits remaining. That's right. To calculate usable IP address we use 2 to the power the number of host bits minus 2. Now look at here very carefully. The formula is very simple. The old formula which is 2 to the power h minus 2 2 the power the number of host bits minus 2. Okay. So we subtract two because the first address is the network address and the last address is the broadcast address.

So um and this cannot be assigned to a device. Okay, this cannot be assigned to a device. So 2 ^ 6 because you have six host bits here. You have six host bits here. So 2 ^ 6 - 2 64 - 2 is 62. So you can if you look at here from 1 to 62 you can use 62. From 1 to 62 you can use 62. It is important to remember that the broadcast address will always be odd numbers and the network address will be even numbers. So if you see here the broadcast address are odd numbers.

The network addresses are uh sorry the network addresses are even numbers except for zero because of course zero can be a network address as well. Apart from that all network addresses must be even number and all broadcast addresses will be odd number. So if you don't find that that means you made a mistake somewhere. So if you don't find that you made a mistake somewhere be careful with that one. Example two, the same thing.

They need eight subnets. Usually in the question, you will find eight subnets with uh let's say 10 IP addresses, eight subnets with 16 IP addresses, something like that. But I'm I'm I want you to learn how to divide learn how to divide. And um you can do the calculation of number of IP addresses as well. So for example, look at here. If the question had been here they said they said four subnets. If the question had been something like this you need four subnets and each subnet will have let's say 500 hosts.

So in this case you cannot use class C. Class C cannot be used because class C can produce only 256 IP addresses and if you subtract two then it will be 254 IPs and that is only one. So you cannot use class C. Okay. So you need to use something like instead of class C class B. Okay. Now let's look at here. We will borrow three bits from the host. Why? Because 2 to the power three 2 the^ 3 is eight. So if you less than that doesn't work.

Borrowing host bits means putting them in the network bits and you need to form the networks. Borrowing bits will increase. We already know that the network can decrease. So there are eight host bits according to what we have here. And there are 24 network bits. So according to the formula if you put 2 to the^ n is the number of borrowed bits. So that will form eight networks. So if you borrow three bits if you borrow three bits out of eight host bits you'll be left with 8 - 3 five host bits.

Okay you'll be left with 8 - 4 5 host bits. So I can type here um if you borrow three bits out of eight host bits. Okay, you will be left with five host bits. Now slash 24 will become slash27 /24 will become slash27 the difference is in the fourth octed of course if you look at here so in this case I borrowed three bits look bit has to be borrowed so if you see here slash24 it is always coming from left to right right left to right so if you are making it /27 it has to be from left to right you cannot borrow from any other it has to be always from the The borrowing bit will has to be always from the left hand side.

Okay? Not from the middle or not from the right. Very important. Write it here. Note, you can only borrow bits from the leftmost bits. Okay? Not from anywhere else. Just see if I can put it somewhere. Okay. Now look at here very carefully. Once you borrowed bits, this is the network portion. This is the network portion and let's say this is the host portion. This is the host portion. That is the network portion and this is the host portion.

Okay. Now if you look at here you these things are going to be changed. Okay. These things are going to be changed. Okay. Okay. So if you look at here we have uh three network bits and um host bits five host bits. You can make changes only in the host portion. There is no change in the network portion. According to the formula of the network address and broadcast address you'll find these are the network addresses. Now look at here uh this is the network address and this is the broadcast address usable.

Now according to the formula of usable host is 2 ^ 5 because you have five host bits minus 2. So you have 30 usable IP address. If you look at here from 1 to 30 usable 33 to 62 usable. So you have 30 usable. The first address is the network address. Last address is broadcast address. This cannot be assigned. So that is why we subtracted two because usually like we had total 32. We could produce total of 32 but we subtracted the network address and broadcast address which is minus2.

So you have 30 usable host. Network addresses should be even numbers according to the previous formula. Broadcast address are odd numbers. Uh if this doesn't match but zero network address could be zero as well. Okay. So it could be zero as well. So it will be zero anyway. Okay. Now, example three. So, we have this and you need to produce 16 subnets. So, we need to borrow four bits. So, if we borrow there are eight host bits, 24 network bits.

If we borrow four bits, it will produce 2 to the^ 4 16 networks. So, you'll be left with four host bits. The cider /24 will become SL28 because you have given four bits from the host portion to the network portion. The difference happened in the fourth octed. Okay. So this is the final time I'm showing you. So if you look at here 8 bits first octed, 8 bits second octed, 8 bits third octed, 8 bits, 8 bits last octed. See usually when it is / 24 it is like that.

Now what's going to happen? You are making it slash 28 that means sorry I got out of the line. So if you are making it /28 that means these eight bits is going to divide four four. Okay you have four network bits and four host bits. That is why you get SL28. Now look at here very carefully. We have borrowed four octets. So this is your the first four is your network portion and the last four is your host portion. Okay.

The first four is your network portion and the last four is your host portion. And you'll see the changes always happen in the first four. Okay? Is always in the first four. The last four always remain the same. Okay? Is always remain the same. And if you look at here, we were able to produce 16. We were able to produce 16. Now if you look at here, if you see first time we borrowed two bits and we are able to produce four.

Last time we borrowed three, we produce eight. Now we are borrowing four, we produce 16. So actually these are the networks that you can produce. So even if you have a requirements where you need 10 networks, you need to go with four 16. So you will have six extra. Okay? You'll have six. You cannot do anything. So it cannot be exact match. It cannot be exact match. It can be an exact match. For example, here now we have an exact match.

So it can be a exact match. But if it doesn't, it is okay as well. Okay? It will not be always an exact match. Now um yep uh we got this one and it will be very important. Previously it was /24 now it will become slash it will become slash28 very important. Okay now look at here this is the network address put. So once you have until this portion so once you until this portion is slash is slash28. Now if you make all the four bits zeros and all the four bits once okay so you will get the network address and you'll get the broadcast address in between are the usable hosts.

Now in the exam if you get a question what is the first usable host? Let me write here first usable host 192 168 dot. So it started with 10. So you need to make sure the 10 10.1 this is the first usable host for one network /26 again if I come to the other network here if I pick from first usable host from the second one is you see again first usable host from the third one is 33 3 they are in different networks. They are in different networks.

So they are the first usable host from different networks. Okay. Now look at here you are left with four uh each subnet has four host bits. Okay. So uh each subnet has subnet has now four host bits. So according to the formula 2 the^ 4 these are the usable IP addresses for each of them network address and broadcast address you cannot assign them to the users. Okay, you cannot assign them to the users.