Subnetting without the Binary Headache!

Annette Smallworth, Senior Instructor for Global Knowledge

A key part of IP networks is subnetting and, when dealing with existing IP networks, an important element is to be able to recognize quickly the subnet that a device is part of. This is a valuable skill to master not only to pass the CCNA exam, which can have many questions on this topic, but also for network troubleshooting.

Traditionally subnetting is taught using the binary method. Although it is important to remember that IP addresses are 32 bit binary numbers, binary mathematics does not come naturally to most human beings.

Looking at the traditional binary method in a little more detail, what would the approach be?

• Convert the IP address from decimal to binary
• Convert the Subnet Mask from decimal to binary
• Delineate the significant bits in the IP address
• Define in binary the subnet address, first host, last host, and broadcast address
• Convert those four binary addresses back to decimal

As you can imagine from the five steps above, this method can be time-consuming and lead to mistakes.

The Subnet Step method is both quicker and easier, and less prone to mistakes because no decimal to binary or binary to decimal conversion is required. With practice, the answer can be calculated without even using pen and paper!

Let's take a look at how the Subnet Step method works by using two example questions.

If the two PCs are on the same subnet, then the device is a switch, but if they are on different subnets, then the device is a router.

So how can we determine if the devices are on the same subnet without having to convert to binary?

The Subnet Step Method

• First determine in which octet the subnet mask ends
  • 255.255.255.248 - In this case the subnet mask ends in the 4th octet
• Now perform a simple calculation: 256 - "mask"
  • To which part of the mask are we referring? The answer is whichever octet the mask ends in; therefore, in this case, as the mask ends in the 4th octet, we subtract 248
• So 256 - 248 = 8
  • Therefore, the "Subnet Step" = 8, meaning subnets with a 248 mask subnets will go up in steps of 8
• Now write down the subnets, starting at 0 and increasing with the value of the "Subnet Step"

    0
    8
    16
    24
    32
    Etc

• We don't need to go any further, because we have now gone past the IP addresses of both PCs in the original question (.22 and .28)
• Recall that the Subnet Address is always the lowest one in the range, so we can determine:
  • PC 204.31.91.22 must be part of the 204.31.91.16 subnet
  • PC 204.31.91.28 must be part of the 204.31.91.24 subnet
• Consequently, as the two PCs are on different subnets, the device in question must be a router!

Job Done! No binary .. No headaches!!

Let's take a look at another question:

• Once again the trick is to first determine the "Subnet Step." This time, the mask ends in a .240 in the 4th octet
  • 256 - "mask"
  • 256 - 240 = 16
  • Therefore, in this example the "Subnet Step" = 16
• Write down the subnet addresses starting at 0 and increasing with the value of the "Subnet Step"

    0
    16
    32
    48
    64
    80
    96
    Etc

• We don't need to go any further since we have gone past each of the IP addresses on the router's interfaces
• Now concentrating on the address on the left hand interface of the router, Fa0/0, 204.30.91.94

   

  .94 must be part of the .80 subnet, and as the "Subnet Step" is 16, the following addresses can now be calculated:

  • The subnet address = 204.30.91.80 (the lowest address in the range)
  • The first available host = 204.30.91.81 (subnet address + 1)
  • The broadcast address = 204.30.91.95 (next subnet - 1)
  • The last available host = 204.30.91.94 (broadcast address - 1)

Having determined the range of addresses for the left-hand side, let's take a closer look at the two PCs connected to the left-hand interface of the router.

The address of PC "A" is within range but notice that PC "B" 204.30.91.99 has been given an address that is outside the range of the available host addresses for this subnet, so it needs to be readdressed.

• Now concentrating on the address on the right hand interface of the router, Fa0/1. 204.30.91.57

   

  .57 must be part of the .48 subnet, and as the "Subnet Step" is 16, the following addresses can now be calculated

  • The subnet address = 204.30.91.48 (the lowest address in the range)
  • The first available host = 204.30.91.49 (subnet address + 1)
  • The broadcast address = 204.30.91.63 (next subnet - 1)
  • The last available host = 204.30.91.62 (broadcast address - 1)

Having determined the range of addresses for the right-hand side, let's take a closer look at the two PCs connected to the right-hand interface of the router.

The address of PC "C" is within range but notice that PC "D" 204.30.91.63 has been given the broadcast address by mistake, so it needs to be readdressed.

Once again ... Job Done! No binary .. No headaches!!

Summary

The "Subnet Step" method is very useful when working with existing IP networks, but as stated earlier, it is still important to understand binary numbers. Binary skills are most often required when deciding which subnet mask to use as part of an IP network design.

When working with existing IP networks though, it would be difficult to beat the speed and simplicity of the "Subnet Step" method. This method is particularly useful in the current CCNA exam where time is of the essence.

About the Author

Annette Smallworth is a Senior Instructor for Global Knowledge specializing in the CCNA and CCNP courses. Annette's first role within networking was as a Field Service Engineer then as a Support Engineer for Digital Equipment Corporation. She became a Network Instructor for Global Knowledge in 1997, delivering Cabletron, Nortel and Cisco courses. Annette is from London, England and in 2006 she transferred from the UK to Global Knowledge US. She currently lives in Washington DC.