Now that you have the basics of how to subnet down, you’ll learn an easier way. If you have learned a different way and it works for you, stick with it. It does not matter how you get to the finish line, just as long as you get there. But if you are new to subnetting, Figure 4.11 will make it easier for you.
FIGURE 4.11 Will Panek’s IPv4 subnetting chart
Subnet Mask | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 | |
255 254 252 248 240 224 192 128 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
1 1 1 1 1 1 1 | 1 1 1 1 1 1 | 1 1 1 1 1 | 1 1 1 1 | 1 1 1 | 1 1 | 1 | 0 0 0 0 0 0 0 | ||
0 0 0 0 0 0 | |||||||||
0 0 0 0 0 | |||||||||
0 0 0 0 | |||||||||
0 0 0 | |||||||||
0 0 | |||||||||
0 | |||||||||
0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
0 = HOSTS 1 = SUBNETS Will Panek’s Chart Power Subnets Hosts
2 | × 2 | = | 4 | –2 | 2 |
2 | × 3 | = | 8 | –2 | 6 |
2 | × 4 | = | 16 | –2 | 14 |
2 | × 5 | = | 32 | –2 | 30 |
2 | × 6 | = | 64 | –2 | 62 |
2 | × 7 | = | 128 | –2 | 126 |
2 | × 8 | = | 256 | –2 | 254 |
2 | × 9 | = | 512 | –2 | 510 |
2 | × 10 | = | 1024 | –2 | 1022 |
2 | × 11 | = | 2048 | –2 | 2046 |
2 | × 12 | = | 4096 | –2 | 4094 |
2 × 13 = 8192 –2 8190
2 × 14 = 16384 –2 16382
2 × 15 = 32768 –2 32766
This chart may look intimidating, but it’s really simple to use once you have done it a few times.
Remember that, on this chart, 1s equal subnets and 0s equal hosts. If you get this confused, you will get wrong answers in the following exercises.
Watch the Hosts column on the lower end of the chart. This represents the number of addresses available to you after the two reserved addresses have been removed. The following exercises provide some examples.
SUBNET MASK EXERCISE 4.1
Class C, 10 Hosts per Subnet
You have a Class C address, and you require 10 hosts per subnet.
- Write down the following:
255.255.255.____
The blank is the number you need to fill in.
- Look under the Hosts column and choose the first number that is larger than 10 (the number of hosts per subnet you need). You should have come up with 14.
- Move across the page and look at the number in the Power column. The power number is 4.
- Go to the top of the chart and look for the row with exactly four 0s (hosts). Find the number at the beginning of the row.
The number at the beginning of the row is 240. That’s your answer. The subnet mask should be 255.255.255.240.
SUBNET MASK EXERCISE 4.2
Class C, 20 Hosts per Subnet
You have a Class C address, and you need 20 hosts per subnet.
- Write down the following:
255.255.255.___
- Look under the Hosts column and find the first number that covers 20. (This should be 30.)
- Go across to the power number (5).
- Go to the top part of the chart and find the row with exactly five 0s from right to left.
The number at the beginning of the row is 224. Your answer should be 255.255.255.224.
SUBNET MASK EXERCISE 4.3
Class C, Five Subnets
Now you have a Class C address, and you need five subnets. Remember that subnets are represented by 1s in the chart.
- Write down the following:
255.255.255.___
- Look under the Subnets column and find the first number that covers 5. (This should be 8.)
- Go across to the power number. (This should be 3.)
- Go to the top part of the chart and find out which row has exactly three 1s (remember, 1s are for subnets) from left to right. Your answer should be 255.255.255.224.
SUBNET MASK EXERCISE 4.4
Class B, 1,500 Hosts per Subnet
This one is a bit harder. You have a Class B address, and you need 1,500 hosts per subnet. Because you have a Class B address, you need to fill in the third octet of numbers. The fourth octet contains eight 0s.
- Write down the following: 255.255.___.0
- Look at the Hosts column and find the first number that covers 1,500. (This should be 2,046.)
- Go across and find the power number. (This should be 11.)
- Remember, you already have eight 0s in the last octet. So, you need only three more. Find the row with three 0s.
You should come up with an answer of 255.255.248.0. This actually breaks down to 11111111.11111111.11111000.00000000, and that’s how you got the 11 zeros.
SUBNET MASK EXERCISE 4.5
Class B, 3,500 Hosts per Subnet
You have a Class B address, and you need 3,500 hosts per subnet.
- Write down the following: 255.255.___.0
- Look at the Hosts column and find the first number that covers 3,500. (This should be 4,094.)
- Go across and find the power number. (This should be 12.)
- Remember, you already have eight 0s in the last octet, so you need only four more. Count for four 0s from right to left.
You should come up with an answer of 255.255.240.0. Again, this actually breaks down to 11111111.11111111.11110000.00000000, and that’s how you got the 12 zeros.
If you get a question that gives you both the hosts and the subnets, always figure out the larger number first. Then, depending on the mask you have decided to use, make sure that the lower number is also correct with that mask.
Now try some more subnet mask exercises using the data that follows:
Class B address | Class B address |
1,000 hosts per subnet | 25 subnets |
Class C address | Class B address |
45 hosts per subnet | 4,000 hosts per subnet |
192.168.0.0 | Class B address |
10 subnets | 2,000 hosts per subnet |
25 subnets
Here are the answers. If any of your answers are wrong, follow the previous examples and try to work through them again.
Class B address | Class B address |
1,000 hosts per subnet 255.255.252.0 | 25 subnets 255.255.248.0 |
Class C address | Class B address |
45 hosts per subnet 255.255.255.192 | 4,000 hosts per subnet 255.255.240.0 |
192.168.0.0 | Class B address |
10 subnets 255.255.255.240 | 2,000 hosts per subnet |
25 subnets 255.255.248.0
Applying Subnetting the Traditional Way
Sometimes subnetting can be confusing. After all, it can be quite difficult to remember all of those numbers. You can step back a minute and take a look at the primary classes of networks and how to subnet each one. Let’s start with Class C because it uses only 8 bits for the node address, so it’s the easiest to calculate. In the following sections, I will explain how to subnet the various types of networks.