Fast IP Subnetting: The 60-Second Cheat Sheet (No Binary Required)

Let’s be honest: Subnetting is boring, but we have to learn it for IT exams, technical job interviews, and of course, when we are working in IT. We may have to resolve IP addressing issues, design a network addressing scheme, advertise a summary route from a large range of IP addresses, or check a routing table.

While subnetting IS boring, it doesn't have to be difficult. By using a simple tickbox formula, you can quickly answer any subnetting question, be it host, broadcast address, subnet, or even design. The more you use this forumual the better you get until you can eventually move it off a sheet of paper and into your brain. Your brain forms an image of the subnetting cheat sheet, ticks the right boxes, and the answer pops out. It might take a few weeks of practice but subnetting in your head is the final goal.

The Subnetting Problem

IT topics are taught by IT people who tend to be very locally minded. They break things down into component parts and expect you to understand things in the same way they do. This is how we end up with websites telling you to break down 192.168.122.147/27 into binary. And so the nightmare begins.

192 → 11000000
168 → 10101000
122 → 01111010
147 → 10010011

The /27 means the first 27 bits are the network portion, and the remaining 5 bits are the host portion.

Network portion (first 27 bits): 11000000.10101000.01111010.100
Host portion (last 5 bits): 10011

If you hope to use this method in an IT exam, I think you will have the time to answer maybe five questions in total before you run out of time. Now imagine trying to use it to help a customer on the phone or in an interview. It just won't work.

In the real world (and in exams), you don't need to understand the binary math behind subnetting every single time. You need a fast way to get the answer. Here is the “Cheat Chart” method I developed to answer any subnetting question in 60 seconds or less—without using a single line of binary code.

The Strategy: Stop Using Binary – Use the Subnetting Cheat Chart

Most textbooks teach you to convert IP addresses to binary, do the ANDing process, and convert back. That works, but it’s slow and error-prone.

Instead, we are going to use a simple grid that you can scribble onto your whiteboard or scratch paper at the start of your exam.

I stumbled on this method back in 2000 when I was trying to learn IP subnetting. I'm sure you have seen other charts, but unless they were invented before 2000, then I'm going to say I invented it! I've seen variations, but they are always making it more difficult than it has to be.

This chart can be hand-drawn or used on lined or grid paper. It's just squares with enough room to put numbers in. These numbers match one octet of an IP address or subnet. That octet is the one we are looking for a host IP address in. For exams or troubleshooting, we need to ignore everything else.

Using this chart, we start by adding numbers beginning at 1 in the top right and doubling each, filling eight boxes. Eight boxes represent one octet. In the left column, we then add 128, which matches at the top-left of the row. We then add 64, then 32, all the way back to 1, getting the pattern below.

Using the Subnetting Cheat Chart

All you do is find the octet that the host is in. I will keep the examples simple here.

Example 1: 192.168.100.100/26

192 is in the Class C range. Address classes really don't matter for exam questions, but we do know that the default subnet mask should be /24 or 255.255.255.0. The mask here is /26, so we have added two to the default. This is all we need. Now we tick across the top row two places and start counting up in that increment until we find where the 100 host would be.

Our subnets increase in increments of 64, and IP subnets can start at 0. In the exam, I'd write out the last octet going up in 64 increments, but here is the complete list of subnets.

192.168.100.0

192.168.100.64

192.168.100.128

192.168.100.192

From these four subnets, can you see which one host 100 would be in? Hint: 0 is too low, and 128 and 192 are too high.

Example 2: 200.168.200.207/27

We've stolen three bits from the standard /24 mask this time. Instead of ticking two places for the last example, this time we tick three, giving us a 32 increment. Now we count up in 32 increments from zero until we get to the subnet host 207 is in. I should have perhaps chosen a host number lower than 207, but let's go with it.

0, 32, 64, 96, 128, 160, 192, 224.

And there we go. Which of these subnets is host 207 in? I'll write it out in full.

200.168.200.192

If you were ever asked to work out the long-hand subnet address from /27, you could use the left column to tick down three to generate 255.255.255.224.

Example 3: 16 Subnets with 14 Hosts Per Subnet From 192.168.1.0

This is the chart's design element that uses the lower portion. All this part needs is the power of two. The first column is the powers of two representing how many subnets. The second column matches, but we need to subtract 2 to account for the subnet address and broadcast address. You can read up on subnetting theory to learn more.

Large numbers of subnets require you to keep doubling more and more, but let's keep this easy, too. You have subnet 192.168.1.0, which is just one subnet and by default will have 254 usable host addresses. The 0 will belong to the subnet, and the 255 number is a broadcast for that subnet, leaving you host numbers 1-254 inclusive.

I need 16 subnets, so I simply tick down the subnets column until I get to that number. Even if they had asked for 15 subnets, I would have to tick as far down as 16 to get enough available subnets.

There are only eight host bits in total for a Class C network, and, with four bits taken for subnets, I have four left for hosts, so I tick down four places for hosts and take two away, as discussed. This gives me 16 subnets, each with 14 hosts. I won't work them all out, but you can if you like for practice.

Subnet 192.168.1.0 hosts 1-14, and 15 is the broadcast address.

Subnet 192.168.1.16 hosts 17-30, and 31 is the broadcast address.

and so on.

The question will ask you what subnet mask you need for this solution. You have stolen four bits, so it would be /28, or if they want it long hand, then tick four down on the upper subnets column to give you 240, or in full 255.255.255.240.

Of course, the questions can be more complicated, but the method never changes.

Learn IP Subnetting

We have a complete beginner to expert IP Subnetting course here if you want to become a subnetting master. It covers all the address classes, binary, hex, and IPv6. By the end, you will be a guru. If you prefer books to videos, then check out our IP subnetting books on Amazon.