16. How to Find the Number of Subnets Valid Hosts

hello and welcome to this tutorial we are going to go over the process the formulas of how to find the number of subnets when we subnet a classful network and also how to find the number of valid hosts in each one of those subnets okay so we've gone over these before and some of the other examples but here we're just going to focus on these exclusively because they're both very important so the idea is for you to walk away from this tutorial being able to use these formulas every time successfully so speaking of the formula is the first one in order to determine the number of subnets it's very simple it's two to the power of the number of subnet bits in the subnet mask likewise in order to find the number of valid hosts in each one of the subnets you're creating the formula is 2 to the power of the number of host bits in the subnet mask minus 2 and we're subtracting the minus 2 at the end here if you remember because we have the network number that we don't assign and we have the broadcast address which does not get assigned to one single host so when we say valid hosts we really do mean the IP addresses in a subnet that we can actually allocate to endpoints like a computer or a router all right so let's run through a few examples ok we're going to look at this network number 192 168 1 0 and we're going to submit it by applying the subnet mask 255 255 255 - 4 0 now that's a slash 28 in prefix notation and we know that because if we look at this subnet mask in binary we can see there are 28 1 bits alright so the first thing we want to do is ask ourselves a question namely is this a class a B or C network 192 168 1 0 well the answer is it's a Class C Network and we know that by looking at the first octet 192 falls within the Class C Network range now that's important because we know a Class C Network has a default subnet mask of 255.255.255.0 which means the first three octets are dedicated to the network portion and we know in subnetting the network portion never changes well it's good to establish that because when we want to find out the number of subnets we can create with this new subnet mask the formula requires us to ^ what well we know what our network bits are we can't touch them that means the remaining ones in the subnet mask have to be our subnet bits so here we have 4 this equals 2 times 2 times 2 times 2 which is 16 so by using this subnet mask we can create 16 subnets 2 to the power of 4 for subnet bits if we move on and now look at the hosts our formula here is 2 to the power of the number of host bits well we stole 4 host bits in this to make this subnet mask so we only have 4 left we plug that in our formula here and in fact it's the same as a subnet we're going to get the same answer 2 to the power 4 equals 16 minus 2 equals 14 don't forget the minus 2 we're subtracting the network number and the broadcast number in order to find the number of valid hosts okay so that's it we've determined that by using this subnet mask we can create 16 different subnets and each subnet is going to have 14 valid hosts let's go ahead and look at another example here we're going to look at 1 5 0.15 0 0 0 that is our network number and the subnet mask we're going to apply to it is here 255 255 255 256 notation as a slash 30 because we're adding up all the 1 bits in the subnet mask and there are 30 of them so just like we did last time let's go ahead and ask ourselves a question is 1 5 0.150 a Class A a Class B or C Network the answer is going to be Class B because 1 5 0 falls within the range of the Class B networks that tells us that the first two octets are dedicated to the network portion and will not be changed so when we go ahead and try to figure out the number of subnets we can create we need to know the number of subnet bits well the remaining subnet bits in this mask are the ones and we can see it is all of these so our formula will be 2 to the power of 14 there are a lot of them this time equals 16384 that's a lot of subnets we're really chopping this this Class B Network into a lot a lot of different subnets let's go ahead and see how many hosts are going to be in each one of those subnets so the formula there is 2 to the number of host bits here there aren't very many there's only 2 left 2 to the power of 2 equals 4 and then we subtract 2 one for the hope one for the subnet number one for the broadcast to give us two valid hosts in each one of these in 84 subnets so here we're creating many subnets but each one is very small in fact we only have two IPS we can actually use to assign to devices in each one of these subnets okay and so that that is the approach you can take in order to answer these two questions whenever you're looking at a classful network and you are going to apply a subnet mask to it in order to then find out how many subnets you're actually going to create and then how big each subnet is going to be in other words how many hosts will be in each one okay well to summarize we really just look at these two formulas and they're very useful in order to answer the questions how many subnets are you creating and how many valid hosts will be in each one so focus on these in practice write out some network numbers you can you can choose them randomly choose one or two class A's B's and C's and then choose a couple different subnet masks like we did and then just walk through the process of each one determine the network bits determine the subnet bits and then determine the host bits as long as you know those three three pieces of information you can use these formulas rather easily in order to answer these questions okay so that is it that is how to find the number of subnets and the number of valid hosts per subnet keep practicing and thanks for watching

Channel: System Engineer

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Length: 8min 3sec (483 seconds)

Published: Mon Apr 24 2017