Some network administrators need to subnet quickly with a minimal amount of math. In this video, you’ll learn about the seven second subnetting shortcut for IP subnetting.
In previous videos, we’ve gone through the process of manually converting an IP address and subnet mask to binary, performing the proper calculations, and converting it back to decimal. We also looked at a shortcut called the magic number method that allows us to very quickly calculate the same information without performing that binary to decimal conversion. This shortcut is the seven second subnetting shortcut, and this is the process that I use when calculating subnetting on an exam.
This is a process very similar in structure to the magic number method, and indeed, most of the shortcuts you find will have some variation of this within them. The part that’s very different to the seven second subnetting is that there is no math involved. You don’t have to perform any calculations to determine a magic number, and everything you need will be predefined in the chart that we create at the very beginning. You’ll find that the only math you end up doing is adding and subtracting the value of 1 to be able to calculate the first IP address and the last IP address.
This is one of many available shortcuts to help you with the subnetting process, and I highly recommend you look at a number of different ones to find one that works for you. This seven second subnetting process works equally well, whether you are online taking this exam and using the online whiteboard, or if you have a physical whiteboard in front of you at a testing center.
The process of calculating the subnet is really about splitting the network into smaller pieces. You may start with a network that has 256 total hosts available, or you might break it up into smaller subnets, such as a dot 128, where you might have two different subnets, a dot 192, which separates it into four different subnets, a dot 244 that separates it into eight individual subnets and so on.
The core to the seven second subnetting process is creating this chart. We will start with a chart that defines the subnet masks. This allows us to very quickly determine what the decimal mask might be if we’re given a CIDR block notation and vice versa. You can see this is very similar to the chart you would create with the magic number method. We’re simply performing the same math on the seven second subnetting.
We also have a Networks column. You would calculate that by starting with the number 2 and doubling it all the way up until 256. And then you have an addresses column, which starts with 128, and you would divide by 2 all the way down until you get to the number 1. And to help with the conversion between the CIDR block notation and the decimal mask, we have a subnet mask in decimal column that we’ll add in as well.
Once you’ve created this chart, you have everything you need to be able to calculate everything necessary for a subnet without having to perform any type of additional math. If you’re like me and you have problems determining what the start of a subnet might be on a subnet that has 16 hosts or 32 hosts, you may want to write out those subnet boundaries into a separate chart so you can very quickly find what those might be during the process of performing your calculations.
For example, a network that has 128 hosts per network would have host boundaries at 0 and 128. A network that has 64 IP addresses per subnet has network boundaries at 0, 64, 128, and 192. And you can calculate similar boundaries for networks with 32 IP addresses, 16, 8, and 4. The seven second subnetting process is a four-step process. The first step is to convert the IP address and the subnet mask to decimal. This is necessary if you’re provided the subnet mask in CIDR block notation, but fortunately, we have a chart that can very quickly translate between CIDR block notation and decimal.
The second step is to determine the subnet address. This is something you may be able to do in your head, or if you’re like me, you’ve written out that second chart that shows you where the delineations are for each individual subnet. That same chart shows us what the broadcast address might be because we can simply look at the last address in that particular range. And from there, it’s very easy to determine what the first and last usable IP addresses might be by adding 1 to our subnet address and subtracting 1 from our broadcast address.
Let’s use this process to subnet 165.245.12.88/24. This slash 24 needs to be converted from the CIDR block notation into decimal notation. You may be able to do this one in your head very easily, but you can also reference your chart by finding the slash 24. You can see that our chart shows the slash 24 as part of that third octet, and it shows that that third octet will have a mask of 255. Everything after that will have a mask of 0. So performing that calculation means that our subnet mask is 255.255.255.0.
Now, we need to calculate the network or subnet address. If the mask is 255, we need to bring that address down, and if the mask is 0, we’re going to simply bring down a 0. So if the mask is 255, we bring down the address. And you can see in the first, second, and third octets, the mask is 255. So we’ll bring down the 165, 245, and 12. And if the mask is 0, we bring down a 0. So in this example, our network address is 165.245.12.0.
To calculate the broadcast address, we look at the subnet mask again. And if it’s 255, we bring down the address number. If the mask is 0, we use 255. In this example, the first, second, and third octets all have 255. So we will bring down our address numbers as 165.245.12. And if the mask is 0, we bring down 255. So in this case, the last octet does have a mask of 0, and we’ll put a 255 in that last octet for the broadcast address.
Now that we have our network address and our broadcast address, it’s relatively easy to calculate the first available IP address and the last available IP address. To calculate the first IP, we take the network address and we add 1. So that would be 165.245.12.1 And the last IP would be the broadcast address minus 1. So that would be 165.245.12.254. There is the seven second subnetting process using our chart and using those simple rules to be able to define the network address, the broadcast address, the first available IP, and the last available IP.
Let’s do another example that doesn’t fall on those simple eight bit ranges of the subnet mask. We’re going to use the same IP address but with a different subnet mask. In this case, it will be 165.245.12.88/26. The first step will be to take the slash 26 CIDR block notation and convert that mass to decimal. If we look at our chart, we can find the slash 26, and we can see that it is the one, two, three, fourth octet. And in that fourth octet, we want to put a 192 decimal. That means that our subnet mask is 255.255.255.192, which is the conversion from the slash 26.
If we look at our chart where it has the slash 26 or the 192, you can see the number of addresses is 64. And since our IP address has an 88, we can look at the different blocks in those 64 ranges to determine where the 88 might be. And in this case, it’s in the range that starts with the block of 64. We’ll follow the same rules we had before. If the mask is 255, we’re going to bring down that address. And if the mask is 0, we’re going to bring down a 0.
In this example, we have three masks that are 255, so we’ll simply bring down the address values for each of those octets. In this last octet, the mask is 192. If we refer to our chart, we can see that we have that range of 64 addresses. We look at our IP address of 88 and determine in which one of those 64 address blocks 88 might be. And it’s in the range that has 64 as its starting point. This is what we’re going to bring down into the fourth octet, is that value of 64.
To calculate the broadcast address, we look at the subnet mask again. And if it’s 255, we bring down the address values. If it’s a 0, we use 255. In this particular case, our subnet mask does not have any zeros. So we need to refer to our chart. Again, we know that this is the range with 64 addresses per subnet. And we know the dot 88 fits into that second block. We know looking at the chart that the next block starts with a 128, which means that the broadcast address must be the one prior to that number. So that would be 127. So the broadcast address is 165.245.12.127.
And of course, to calculate the first usable IP address, we add 1 to our network address. So our first IP is 165.245.12.65. The last IP address is determined by taking the broadcast address and subtracting 1. So that would be 160.245.12.126.
We can perform this same shortcut with any IP address and subnet range. Let’s take the example of 160.245.12.88/20. If we look at this address, we take the slash 20 and convert it to a decimal subnet mask. We can determine this by finding the slash 20 in our chart. It is in the third octet. And that third octet will be a 240. So everything to the left of that will be 255, and everything to the right will be zeros. So our subnet mask of a slash 20 is the same as a 255.255.240.0.
You also notice in this chart that the number of addresses in each of these subnets will be 16 addresses. And if we look at our chart, we know that we have an address that has 165.245.12.88, so we need to determine where the dot 12 might be in our list. That dot 12 fits between the 0 and 16, so it’s this very first subnet that we’re going to look at in this particular range.
Let’s calculate our network address. If the mask is 255, we bring down the address, and if the mask is 0, we use 0. In this case, we have a 255 in the first two octets and a 0 in the last octet, so we simply bring down each one of those. Since we’re looking at the range that has the dot 12, and we know that there are 16 hosts on each of these subnets, then that dot 12 is in that range between 0 and 16, and the first number being 0 means that that is our network address. And so we’ll bring that down as our third octet. This means that our network address is 165.245.0.0.
To calculate the broadcast address, we look at every place there’s a 255, and we bring down that address. So that would be our first two octets. And if there is a 0 as part of the subnet mask, we use 255, and we’ll bring down that 255 for the fourth octet. Since we have a 240 as the subnet mask in that third octet, we need to look at our chart again and determine what number we would bring down. Since we know there are 16 addresses in each of those blocks, we know that the next subnet will start with a 16. So one number prior to that 16 would be a 15. And the broadcast address for this subnet is 165.245.15.255.
Now, we can calculate the first available IP address by looking at the network address and adding 1. That would make it. 165.245.0.1. And we can calculate the last available IP address by subtracting 1 from the broadcast. And that would be 165.245.15.254.
Let’s go through the process again with an IP address of 18.172.200.77/11. The first thing we’re going to do is convert that slash 11 to a decimal mask. And if we look at our chart, we’ll find the dot 11 is in the second octet, and it has a value of 224. So our subnet mask is going to be 255.224.0.0.
On the same line as that subnet mask is the number of addresses. And you can see in this particular example, there are 32 addresses per subnet. Since that slash 11 is in the second octet, we’re going to look at that IP address to determine where it fits in those blocks of 32 IP addresses. So if we were to look at our chart of these 32 address blocks, we know that 172 fits in this range between 160 and 191.
So to calculate the network address, if the mask is 255, we’re going to bring down the 18 that’s in that block. And if it’s 0, we’re going to bring down zeros. And since this block starts with a 160, our network address is 18.160.0.0.
To calculate the broadcast address, if the mask is 255, we bring down the address, and if the mask is 0, we use 255. This means in the first octet, we’ll simply bring down the 18. And in the third and fourth octets where the mask is 0, we’ll write 255. In this third octet, we’ll go back to our chart. And we know that’s the range between 160. And in this case, the next range starts at 192. Therefore, the broadcast address for the previous range would have to be 191.
And to calculate the first and last available IP addresses, we will add 1 to the network address and subtract 1 from the broadcast address, meaning the first IP is 18.160.0.1, and the last available IP address is 18.191.255.254.
Let’s perform one more calculation to see how well we’ve captured the shortcut. We’ll use the address of 18.172.200.77/17. If we go to our chart, we can find the slash 17 as part of the third octet, and it matches that 128 value as the decimal part of the mask. So if we were to write out the mask, it would be 255.255.128.0.
If we were to look at our chart, that slash 17 is on the same line as blocks of 128 hosts. So we know that there were 128 IP addresses in each one of these subnets. So let’s calculate from there what the network address might be. If the mask is 255, we will bring down the address value. And if the mask is 0, we will use 0.
To calculate the broadcast address, we look at the subnet mask, and if it’s 255, we bring down the address. And if the mask is 0, we use 255. In this example, we have subnet mask of 255 in the first two octets, so we’ll bring down those address values. And we have a 0 in the fourth octet, so we’ll use 255.
In this example, that value of 200 is in that third octet, which puts us in the second block starting with 128 and ending with 255. So in this case, the broadcast address would be the last address in that block, which is 255, meaning your broadcast address is 18.172.255.255.
And of course, to calculate the first IP address, we add 1 to the network address, making it 18.172.128.1. And we calculate the last IP by subtracting 1 from the broadcast address, making the last IP 18.172.255.254.
As you can probably tell, the key to this seven second subnetting process is being able to reference that chart. So you might want to try writing this down in a chart if you’re going to be in a testing center, or you may want to try typing it into a notepad or wordpad to see how quickly you can type it in if you’re going to be taking the exam online.
If you go to a testing center, you may find that the dry erase pens that they use may not allow you to make a very easily readable chart, so you may want to bring your own pen. Make sure you check with the testing center that it’s OK to bring your own fine tip dry erase pen into the test itself. And whether your shortcut is the seven second subnetting process, the magic number process, or any other process, make sure you find the shortcut that works best for you and decide to use that for your exam.