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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing

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IP Variable Length Subnet Masking (VLSM)
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IP Subnetting Step #1: Requirements Analysis
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IP Subnetting: Practical Subnet Design and Address Determination Example

When educators ask students what they consider to be the most confusing aspect in learning about networking, many say that it is IP address subnetting. While subnetting isn't all that difficult in concept, it can be a bit mind-boggling in part due to the manipulations of binary numbers required. Many people understand the ideas behind subnetting but find it hard to follow the actual steps required to subnet a network.

For this reason, even though I explained the concepts behind subnetting in detail in the previous section, I felt it would be valuable to have another section that provides a step-by-step look at how to perform custom subnetting. This section divides subnetting into five relatively straight-forward stages that cover determining requirements, making the design decision of how many bits to use for subnet ID and host ID, and then determining important numbers such as the subnet mask, subnet addresses and host addresses.

My focus in this section is on showing the practical “how” of subnetting. The topics here work through two examples using a Class B and a Class C sample network to show you how subnetting is done, and I am explicit in showing how everything is calculated. This means the section is a bit “number-heavy”. Also, I try not to duplicate conceptual issues covered in the previous section, though a certain amount of overlap does occur. Overall, if you are not familiar with how subnetting works at all, you will want to read that section first. I do refer to topics in that section where appropriate, especially the summary tables. Incidentally, I only cover conventional subnetting here, not VLSM.

This section may serve as a useful refresher or summary of subnetting for someone who is already familiar with the basics but just wants to review the steps performed in subnetting. Again, always bear in mind that subnetting is based on the older “classful” IP addressing scheme, and today's Internet is classless, using CIDR.

Background Information: If you are not familiar with binary numbers, binary-to-decimal conversion and masking, and you didn't take my advice in preceding sections to brush up on these concepts using the background explanation of computational math, you really want to do that now.

Note: If in reading this section you find yourself wanting to do binary-to-decimal conversions or binary math, remember that most versions of Windows (and many other operating systems) have a calculator program that incorporates scientific functions.

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Previous Topic/Section
IP Variable Length Subnet Masking (VLSM)
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IP Subnetting Step #1: Requirements Analysis
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Version 3.0 - Version Date: September 20, 2005

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