IPv4 Subnetting: Subnetting in Your Head

Subnetting is an important skill for any network engineer, and being able to do it quickly and accurately in your head can be a valuable asset. While subnetting can seem daunting at first, with practice and the right approach, it can become second nature. In this blog post, we will provide step-by-step instructions and some IPv4 subnetting examples presenting the easiest way to learn to subnet in your head.

Step 1: Understand the Basics of Subnetting

Before you can start subnetting in your head, you need to have a solid understanding of the basics. This includes understanding IP addresses, subnet masks, and how they are used to create subnets. It’s also important to know the different classes of IP addresses (A, B, and C) and their default subnet masks. You can find plenty of online resources to help you review these concepts if needed.

Step 2: Memorize the Powers of 2

Subnetting involves working with binary numbers, so it’s important to have a good understanding of the powers of 2. Memorizing the first eight powers of 2 (2^0 through 2^7) can be particularly helpful, as they are used frequently in subnetting. Here they are: 1, 2, 4, 8, 16, 32, 64, 128.

Step 3: Practice with Simple Examples

To start practicing subnetting in your head, it’s best to start with simple IPv4 subnetting examples. For instance, try subnetting a Class C network (192.168.1.0) into four subnets. First, determine the number of bits needed to create four subnets (two bits, as 2^2 = 4). Next, determine the new subnet mask by adding two bits to the default subnet mask for a Class C network (/24). The new subnet mask is /26 (24 + 2 = 26). Finally, determine the subnet ranges by starting with the original network address and adding the subnet increment (64) for each new subnet. The four subnets would have the following ranges: 192.168.1.0 – 192.168.1.63, 192.168.1.64 – 192.168.1.127, 192.168.1.128 – 192.168.1.191, 192.168.1.192 – 192.168.1.255.

Step 4: Use Shortcuts

As you get more comfortable with subnetting, you can start using shortcuts to make the process even faster. For example, you can memorize the subnet mask increments for each power of 2. For example, for a Class C network, a /24 subnet mask (255.255.255.0) represents 256 IP addresses. A /25 subnet mask (255.255.255.128) represents 128 IP addresses, a /26 subnet mask (255.255.255.192) represents 64 IP addresses, and so on. By memorizing these increments, you can quickly determine the number of subnets and hosts for a given subnet mask.

Step 5: Practice, Rinse, Repeat

Like any skill, subnetting requires practice to master. The more you practice subnetting in your head, the more familiar and comfortable you will become with the process. There are many online resources available that offer practice problems and quizzes to help you improve your subnetting skills. Subnetting is an important skill for any network engineer, and being able to do it quickly and accurately in your head can be a valuable asset in the field as well as when sitting for the CCNA exam. As with the exam, in the field there is rarely time to consult a subnet calculator. Also, your network engineering credibility decreases dramatically if you cannot comfortably talk subnets with other network engineers. By following these steps and practicing regularly, you can become a subnetting master in no time.