Subnetting by the Numbers

Subnetting By The Numbers

This paper is an attempt to assist instructors in teaching IP v4 subnetting. It is not intended to be a student study guide. However, use it as you will. After a student understands the binary and decimal numbering systems and the conversion between the two, I step off into the deep, dark world of IP subnetting. To start with, I advise the students that this is not rocket science. All they need is their CD calculator (actually they need a little more but I kind of fudge here). You know, what a CD calculator is don't you? Country Digital calculator! I hold up my fingers and count the digits (without thumbs) from one to eight. That is all they need to know about math to do IP subnetting. They don't even have to use their thumbs or take off their shoes! This seems to distract and take a lot of the fear away at the beginning. I use several little phrases and stories to enable a "mind hook" for students to associate a process step with an image. It seems to help.

This is just a primer to help get down the mechanics in order to see the whole picture. Students MUST grasp the concepts of bits and subnetting to understand IP routing and routing access lists. If this works for you or your students, great, if not, then maybe a part or some parts will help. If not, then contact me to help me to do it better. I do not permit the use of electronic calculators or spreadsheets for any subnetting. My thoughts are that a student must understand what is happening with the whole picture of networking in mind. IP subnetting IS the basic seed of understanding networking.

The important points here are the concepts of subnetting, the whole network idea, gateways, and the process to solve problems. Sometimes it is best to gather all the information we can about a problem and use that information to answer a question or questions. This is a process I use to gather everything we can and then solve the problem. The five-step beginning is to get the student started. One of the most difficult things for students is remembering what to do first and where to start. With the five-step process a student can have needed information to solve most problems. This step by step process will enable beginning students to document a lot of information when they are given a small amount of information, such as: host IP address or network/subnet IP address, subnet mask, number of network bits (CIDR), or needed subnets and/or hosts. The process will vary depending on what is given in the problem, but I always strive to connect PLANNING with the process. I have found it best to keep the problems which are solved by first giving the number of subnets and/or hosts as the last type of problems because they set up the planning discussion. This way I can show WHY they must be able to figure subnets. Which relates it to the real world. Students need to know how they can find the valid range (all in the same subnet) to place into the DHCP server, or 80% of the valid range in one DHCP server and 20% of the valid range in a second DHCP server.

Short cuts are not used to describe the process until well into the lesson(s). Students explain solutions to the class by solving problems at the board in front of the class. This is both individual and teamwork. I have found teams of two work best, one student does the board work and the other student explains the work to the class as they coordinate the solutions. They rotate positions next time around.

The paper calculator (legal cheat sheet) including Zorro, ABCs, 2sies, and range tables (okay, they're corny, but memorable - you'll see) should be built at the beginning of each class. This is practice for test and job performance time when they will need it. I believe students also understand the relationships between the parts of IP subnetting better as they make