Subnetting - A Popular Exam Topic

Subnetting is often one of the most difficult concepts to grasp. But it is one that is important to understand for both on the job and exam success. At first the concept may seem mindboggling but once you’ve done it a few times, it really becomes less complex than it first appears to be. But it’s definitely a popular topic on the Microsoft exam and if you are working towards an MCSE or MCSA designation, you gotta know it!

When it comes time to take an exam, be prepared to encounter at least one (if not several) exam questions related to subnetting. You should be able to perform basic subnetting for a given scenario. For example, if you are presented with a scenario outlining the required number of subnets and the expected number of hosts per subnet, you must be able to select the appropriate subnet mask.

So when it comes to answering the subnetting questions, here are a few tips to keep in mind. Be sure to read the scenario carefully. In order to choose the correct subnet mask, you must take into consideration any expected growth. A scenario may tell you that 6 subnets are required but if you read carefully, it may also mention that there are plans to add 6 more in the near future. Which means you’ll need to choose a subnet mask that will allow for at least 12 subnets.

You also need to be able to determine the number of subnets that can be created for a given subnet mask as well as the number of hosts per subnet. There is a simple formula you can use to determine this. For example, the subnet mask of 255.255.255.224 would allow for 8 subnets. You can determine this by using the following formula: 2 ^ x where x is the number of host bits used for the subnet mask. Since 224 uses 3 bits, using the formula means 2 ^ 3 equals 6 subnets.

You can use the same formula to determine the number of hosts where this time x equals the number of bits remaining for the host ID. Since 3 bits are being used for the subnet mask, this leaves 5 bits remaining. 2 ^ 5 equals 32 which means each subnet can have a maximum of 32 hosts. If you can remember that formula, you will be able to determine the number of subnets and hosts that a specific subnet mask will allow for.