if you've been with us with the videos just prior to this one then you've learned a lot about subnetting masks cider block notation and other important fundamentals when it comes to subnetting Via IP version 4 all of that work has brought us to this point where we can start to answer questions like this one here's a network layout and it says here's your IP address assignment we've been given the IP address of 192.168.1.0 /24 and this says we need an IP addressing scheme with more than one network address that can support 40 devices per subnet that is the optimal number we're looking for and we need to determine how to subnet this network address to be able to get 40 devices per subnet now you could of course write out all of the possibilities in decimal and binary and be able to calculate what the differences are with the number of networks and the number of hosts per Network for example if you look at the default subnet Mas that we were given which is 255.255.255.0 you can see this is the binary representation of that subnet mask and the cider notation which is a sl24 that gives us one single network with 254 hosts per network if we borrow one bit from the host side so that we are 255 255.255 1228 as our subnet mask or a sl25 we would be able to create two networks with 126 hosts per Network we are getting closer to that number of 40 let's keep going by borrowing another bit so that our subnet mask is 255.255.255.192 or sl26 this allows us to have four networks with a total of 62 hosts on each of those subnets this is certainly getting close to 40 but let's go one more step to see if we're getting even closer to that 40 Value The Next Step would be to borrow one more bit from the host side so now we're borrowing three subnet bits which means our subnet mask is 255 255 255 224 or a sl27 if we Subnet in this way we can have a maximum of eight networks but it would only allow for 30 hosts per Network this means that the optimal subnet m ask if we're looking for a total of 40 devices per subnet would be the next one up or the sl26 which would give us four networks and a maximum of 62 hosts per Network now we can certainly speed this process Along by performing a calculation based on Powers of two let's look to see how we might do that we have a starting IP address of 192.168.1.0 and a subnet mask of 255.255.255.192 to and based on this information we can show the network address in the subnet mask in binary and you'll notice in the subnet mask we have the default number of bits for that address plus the two extra bits that we borrowed from the host side visually you can see where the 24 subnet bits might be the two extra bits we borrowed for the subnet and the six bits that are left over on the host side if we were to now calculate the number of subnets and the number of hosts per subnets using this powers of two formula we would take 2 to the 2 power because the second power would be two bits and that would be a total of four networks we would determine the number of hosts per subnet by taking those six host bits that are left over at the end and calculating two to the 6th power minus 2 which would be 64 minus 2 or a total of 62 host per subnet this is a relatively straightforward way to calculate the number of subnets and the number of hosts per subnet but it requires you to perform a binary conversion and then count the number of bits to be able to calculate what that total might be at the end there are some shortcuts that we can use and in this video we're going to look at a shortcut that uses the magic number method these calculations are intended to give us four separate pieces of information first we need the network address or subnet ID this would be the first address in the subnet we also need a broadcast address this is the last address in the sub subnet we need the first available host address this is an address that we can assign to a device on this subnet and then we need the last available host address we need to be able to calculate these as quickly as possible now obviously we could calculate this by converting everything to Binary looking at the subnet bits and then calculating for every possible scenario what this particular Network address might be and the subnet mask for each one of these and for this particular IP address where we determine the were four separate subnets these are all the calculations that we would need to do to be able to calculate all of that information for all four of those subnets you can see looking at this screen that there are a lot of separate binary to decimal conversions you'd have to do just to calculate those specific details for all four of those subnets we can simplify this process by using the magic number shortcut this allows us to perform subnet calculations very quickly and very often we can do all these calculations in our head there's still a little bit of math involved with the magic number method but it's limited to powers of two and some very simple addition and subtraction you could even create a few charts that might help you with the process but once you get accustomed to using this method you'll find that you memorize very easily what's in the charts you may not have to write them down at all during your exam one chart that might help you is a chart that easily converts between a cider block notation and the decimal subnet mask notation ation you can see an example of this might be to use a sl9 is 25128 z.0 sl10 is 2551 192.000 and so on you will notice that there are some patterns after a while for example a sl9 has the 128 as the number that is a bit different than a zero or a 255 and a sl17 also has a 128 so we could write this chart in a simplified form that shows a SL 9/7 and a sl25 use the 128 as the decimal value in the magic number method we have a number of different tasks that we perform based on the octet that we're working with so we might want to modify this cider to decimal chart to have something that's a little bit different for example if you wanted to see The Cider notation for interesting octet 2 you can see it ranges from a sl9 through sl16 the cider block notation for interesting octet 3 would be sl7 through sl24 and The Cider block notation for interesting octet 4 ranges from sl25 to sl30 you notice that we don't use /31 by default because it doesn't leave any room for any hosts on the subnet and we don't use a sl32 because that leaves zero bits available for any host for each of these columns with the cider block notation you'll notice there is a magic number and in a moment we'll learn how to use that magic number value we can also put in this chart the subnet mass for an interesting octet and you can see those Mass values are also listed as well starting at 128 and working all the way up to 255 optionally you could also create a Host range chart where you can take an address block and then separate out what the differences are for that particular block for example an address block of 128 splits the network into two different pieces 0 through 127 and 128 through 255 an address block of 64 hosts per block would be 0 through 63 64 through 127 and so on and you can continue this process with a host block of 32 hosts 16 hosts 8 hosts and so on so let's see how the magic number method works it's a series of steps and once you go through these a couple of times they become very second nature the first step is to convert the subnet mask to decimal if it's not in decimal already from there we need to identify the interesting octet we'll talk about the interesting octet in just a moment we then need to calculate the magic number which is 256 minus the value of the interesting octet we could then calculate The Host range based on that identify the network address which would be the first address in that range and then identify the broadcast address which would be the last address in that range so now let's take an IP version for subnetting problem and answer this problem using the magic number method we'll start with this IP address of 165 24577 14 and we've been given a subnet mask of 255.255.255.0 do0 we can then also fill in the IP address that we been given which is 16524 7714 from here we can start to make some decisions about what to put into this chart for example if the mask is 255 you simply copy the IP address down so in this case the mask is 255 in the first octet so we're going to copy that down which means our subnet ID will be copied down with the same number that's in the IP address row let's perform that same task again with the second octet it is also 255 so we're going to copy that through this 245 then will be brought down into the subnet ID row we no longer have any octets with the 255 so our next step will be to look at any octets where the mask is zero and if the mask is zero we bring that zero down to the subnet ID Row in this case our last octet does have a zero so we're going to copy that zero all the way down and put it into the subnet ID Row in the last octet in our example example the subnet mask in the third octet is a 240 that is obviously not zero and not 255 so that octet is our interesting octet now that we know the interesting octet we can use that to calculate the subnet ID value for that particular octet to be able to do that we need to subtract the interesting octet subnet mask from 256 in our interesting octet the subnet mask is 240 so if we subtract 240 from 256 we have the value of 16 this means our magic number is 16 and there would be 16 hosts available on that particular subnet if we put our chart at the bottom that shows the 16 different hosts that you would have and the delineation between each one of those subnets you can see that we have a range between 0 and 15 16 through 31 32 through 47 and so on now if we look at the IP address we can see the IP address is 7 7 and if we look on our chart we can fit the 77 IP address in the range between 64 and 79 since we're trying to determine the subnet ID we take the very first number in that particular range which would be 64 and we add that 64 into that subnet ID in our interesting octet so for the IP address 165 24577 14 that has a subnet mask of 255.255 240. the subnet ID is 16524 64.0 of course not only do we need to know the subnet ID but we also need to know the broadcast address to calculate the broadcast address using the magic number method it's a very similar process we use the same chart with the same subnet mask and the subnet ID but at the very bottom we have broadcast address instead of subnet ID the rules for determining the broadcast address are very similar to the rules we used for the subnet ID if the subnet mask is 255 we copy the subnet ID all the way down into the broadcast address row we have two of our octets that show a 255 so we'll do this for the first and the second octet here's where the rules are a little bit different when calculating the broadcast address versus calculating the subnet ID if the mask is zero we're going to write into that column A 255 and of course if the net mask octet doesn't contain a zero and it doesn't contain a 255 then that octet is our interesting octet this will obviously be the same interesting octet when we're calculating the broadcast address as it was when we were calculating the subnet ID to calculate the magic number we perform the same process that we did earlier we take 256 we subtract from that the value that's in that interesting octet mask leaving us with a magic number of 16 to calculate the broadcast address for that interesting octet we take the subnet ID and we add the magic number and subtract one so in this example we use 64 because that is the number in our subnet ID we add 16 which is our magic number we subtract one and that leaves us with 79 so in this example the broadcast Address is 165 24579 255 now that we know the subnet ID and we know the broadcast address calculating the first available IP address and the last available IP address is relatively straightforward to calculate the first host we would take the subnet ID and add one so the first address on the subnet is 165 245 64.1 to find the last usable address on the subnet we take the broadcast address and subtract one so the last usable address on this subnet is 16524 7925 54 and that is the magic number process without doing any type of conversions between binary and decimal we were able to calculate the subnet ID the broadcast address the first available IP address and the last available IP address let's see how well we can use this magic number method with another example we'll use the IP address of 101801 122. 244 and the subnet mask is 25524 0 we will first fill in our chart so we'll put our subnet mask in that mask row and we'll put our IP address in the IP address row we'll now look at our subnet mask and everywhere there's a 255 we will use that to copy down the IP address so in this case in the first octet we'll copy down that value of 10 every place there is a zero we're going to copy down the zero and put that into the subnet ID row and the third and the fourth octets both have a zero since we don't have a Zer or a 255 in this second octet that means that this octet is our interesting octet to calculate the magic number we use 256 subtract the value that we have in that subnet mask range which means we use 256 - 248 so in this example our magic number is 8 this means that each subnet has a total of eight IP addresses that can be assigned including our subnet ID and our broadcast ID and we've put the chart at the bottom that separates this into those multiples of eight now we need to find the IP address that is the starting for this particular block if we look at our interesting octet we know that the IP address is 180 so if we look at our chart we can find that 180 which is in the block starting with 176 since that starts with 176 we can simply fill that in which means the subnet ID for this example will be 10.176 0.0 now we need to find the broadcast address for this example we'll use a similar process we did before where if we had a 255 in the list we're going to copy and bring down that subnet ID so the first octet will simply copy down the number 10 anywhere that we have a zero we're going to copy down and put a 255 and we have zeros in both our third octet and our fourth octet as we already know the second octet is our interesting octet so we're going to perform our calculation taking 256 subtracting 248 from that meaning our magic number is 8 this is obviously the same magic number we calculated earlier to calculate the broadcast address we need to take the subnet ID add the magic number and subtract one so the example here would be 176 Plus 8 since that's our magic number and if we subtract one that leaves us with a number of 183 we'll add this to our broadcast address row which means the broadcast address for this example is 10.1 18325 5255 now that we've calculated the subnet ID and the broadcast address it's relatively easy to determine the first host ID because we take the subnet ID and add one so the first available host on the subnet is 10.176 do1 to determine the last address we take the broadcast address and subtract one so in this case it would would be 10183 255.255 hopefully now you can see that the magic number method is a bit faster than going through the steps of converting to Binary and then back to decimal again but there are some ways to make this process even more efficient if you've already created a chart that shows all of the possibilities of an interesting octet cider block notation and has the magic number predefined along with the subnet mask for the interesting octet you can perform this in entire process in one single chart let's take the example of 17216 24233 sl27 if we look at our predefined chart we know that sl27 falls into the cider block notation for interesting octed 4 this means that octet 4 will have a decimal value of 224 so if we wanted to write out the entire subnet mask it would be 255.255.255.0 because 22 24 is in that interesting octet number four the process for calculating the magic number is exactly the same as it was before we take 256 we subtract that subnet mask in the interesting octet which in this case is 224 which means our magic number is 32 there are 32 available addresses on each subnet of this particular IP address and subnet MK combination here's our chart which is written out with 32 numbers in each of those sections and if we look at our IP address that has a 133 we can move forward to where the 133 would be and it is in the block that starts with a 128 since we know it starts with a 128 that means that we can now change our fourth interesting octet to be that value as the subnet ID so the subnet ID in this example would be 17216 24218 if you recall from our earlier calculations to be able to calculate the broadcast information we need to take the subnet ID value in the interesting octet add that to the magic number and subtract one so we'll take 128 We'll add 32 to make 160 we'll subtract 1 and that makes 159 so the broadcast address on the subnet is 17216 24259 now that we know the subnet ID and the broadcast we can calculate the first available IP address and the last available IP address we add one to the subnet ID to determine the first IP so the first IP would be 17216 2421 129 if we subtract one from the broadcast address we would get 17216 24218 so there is the faster way to be able to calculate our subnet ID our broadcast address the first IP address and the last IP address using the e