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Chapter 3

IP Subnetting and Variable Length Subnet Masks (VLSMs)

How to Create Subnets
To create subnetworks, you take bits from the host portion of the IP address and reserve them to define the subnet address. This means fewer bits for hosts, so the more subnets, the fewer bits available for defining hosts. Later in this chapter, you’ll learn how to create subnets, starting withClass C addresses. But before you actually implement subnetting, you need to determine your current requirements as well as plan for future conditions.

Before we move on to designing and creating a subnet mask, you need to understand that in this first section we will be discussing classful routing, which means that all hosts (all nodes) in the network use the exact same subnet mask. When we moveon to Variable Length Subnet Masks (VLSMs) I’ll discuss classless routing, which means that each network segment can use a different subnet mask.

Follow these steps:
1.

Determine the number of required network IDs: One for each subnet One for each wide area network connection

2.

Determine the number of required host IDs per subnet: One for each TCP/IP host One for each routerinterface

3.

Based on the above requirements, create the following: One subnet mask for your entire network A unique subnet ID for each physical segment A range of host IDs for each subnet

Understanding the Powers of 2
Powers of 2 are important to understand and memorize for use with IP subnetting. To review powers of 2, remember that when you see a number with another number to its upper right(called an exponent), this means you should multiply the number by itself as many times as the upper number specifies. For example, 2 3 is 2 × 2 × 2, which equals 8. Here’s a list of powers of 2 that you should commit to memory: 21 = 2 22 = 4 23 = 8

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24 = 16 25 = 32 26 = 64 27 = 128 28 = 256 29 = 512 210 = 1,024 211 = 2,048 212 = 4,096 213 = 8,192 214 = 16,384Before you get stressed out about knowing all these exponents, remember that it’s helpful to know them, but it’s not absolutely necessary. Here’s a little trick since you’re working with 2s: each successive power of 2 is double the previous one. For example, all you have to do to remember the value of 29 is to first know that 28 = 256. Why? Because when you double 2 to the eighth power (256), you get29 (or 512). To determine the value of 2 10, simply start at 28 = 256, and then double it twice. You can go the other way as well. If you needed to know what 2 6 is, you just cut 256 in half two times: once to reach 27 and then one more time to reach 2 6.

Subnet Masks
For the subnet address scheme to work, every machine on the network must know which part of the host address will be used as thesubnet address. This is accomplished by assigning a subnet mask to each machine. A subnet mask is a 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address. The network administrator creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer to thenetwork or subnet addresses. Not all networks need subnets, meaning they use the default subnet mask. This is basically the same as saying that a network doesn’t have a subnet address. Table 3.1 shows the default subnet masks for Classes A, B, and C. These default masks cannot change. In other words, you can’t make a Class B subnet mask read 255.0.0.0. If you try, the host will read that addressas invalid and usually won’t even let you type it in. For a Class A network, you can’t change the first byte in a subnet mask; it must read 255.0.0.0 at a minimum. Similarly, you cannot assign 255.255.255.255, as this is all 1s—a broadcast address. A Class B address must start with 255.255.0.0, and a Class C has to start with 255.255.255.0.

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IP Subnetting and Variable...