2.7. Identifying the Purpose of Subnetting

If you use a classful network address with a default subnet mask, then you have one and only one network. This might be fine if you only have one location, but what if you have many locations? Do you have to be assigned another classful network address for each location? You do unless you can divide your network into multiple networks so that you can use the network addresses in a more efficient manner. In addition, if you are using two different technologies, such as Ethernet and Token Ring, then you would need to maintain them on two separate net-works for security and control. These are the main reasons for subnetting. In this section, we discuss subnetting in greater detail.

2.7.1. Critical Information

You should understand the purpose of subnetting a network. In addition, you should know the procedure that is applied to a registered address to create subnets. Finally, you should be able to determine the number of subnets and the number of hosts per subnet that are created by changing a subnet mask from the default to a custom subnet mask.

2.7.1.1. Subnetting Basics

Subnetting allows an organization to have one registered public network address and yet use many addresses from that address for each of its locations. This is accomplished by changing the subnet mask to something other than the default. Based on the subnet mask assignment, you can create more networks with fewer hosts in each network. For example, a Class B network address such as 174.23.0.0 would have a default subnet mask of 255.255.0.0. This means that 16 bits are used for the network and 16 bits can be used for hosts. With 16 bits used for hosts, you can have a maximum of 65,534 hosts. You can determine this by the formula 2ⁿ−2, where n is the number of bits. In this case 2¹⁶−2 =65,534.

Now if you have a building that will hold 65,534 users, then you are all set, but chances are your users are located in more than one geographical location. Suppose you have six locations and you have thousands of users at each location. You can't have the same network address on both interfaces of the router that connects them, so you need more networks.

Since you have 16 bits for hosts, why not "borrow" some of those to create some subnets? Using a related formula to the one we just discussed, you can determine the amount of bits that you need to borrow to create six subnets by solving for n in the equation 2ⁿ−2≥6. Solving for n, you get the number 3 or higher. This means that if you borrow the first three bits after the network bits, then you can use them to create six new subnets. Looking back at Table 2.4, you can see that the total value of the first three bits is 224 (128+64+32). This means that if you change the default subnet mask to a custom subnet mask of 255.255.224.0, you will be able to create six subnets. Since each of the subnets can now use the remaining bits, you can have 2¹³−2 hosts in each subnet, or 8,190 hosts per subnet.

2.7.2. Exam Essentials

Describe the main reasons that subnetting is used. Subnetting is used to create more network addresses to provide greater control and greater security, and to use one registered network address for multiple geographical locations of an organization.

Understand how to use 2ⁿ−2. Be able to use the formula 2ⁿ−2 to determine the appropriate subnet mask that can be used with a specified classful network address to create a certain number of subnets with a specified number of hosts in each subnet.