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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 with Class 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 move on 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: Determine the number of required network IDs: One for each subnet One for each wide area network connection Determine the number of required host IDs per subnet: One for each TCP/IP host One for each router interface 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, 23 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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