I’ve been asked to help people understand subetting a few times, so I thought I’d document it here to give me a place to point them to in future. Subnetting is explained in many places, with a large variety of methods. The key to working out valid subnets however is simply a matter of understanding the binary maths behind it. This guide explains a method to quickly do subnet maths, without any tricks. The guide will help you in calculating subnet boundaries and valid host address fast enough for use in a Cisco CCNA or ICND1 exam.
The method I use when subnetting starts with the table below. The first row shows the decreasing powers of 2, or to put it simpler - starting with 1 on the right, keep doubling the number 8 times, you should end up with 128. The row under it is easily calculated by adding up the numbers in the row above. For example, the 2nd cell from the left on the 2nd row (192), is calculated by adding 128 and 64 from the row above. The 3rd number (224) is 128+64+32, or to simplify, it’s the previously calculated 192, added to 32. This explanation might sound a little complicated, but that’s just because I wanted to be sure you understand the table below fully. The numbers in this table will become second nature to you after a little practice.