The wildcard mask will be used to identify which hosts belong to which network. Next, calculate the wildcard masks of each subnet. From this, the number of bits to borrow can be determined. To begin classful subnetting, identify the number of subnets needed and the class of the network. Classful subnetting follows the traditional IP addressing structure of A, B, and C classes.
This is achieved by borrowing bits from the host portion of the IP address to create a subnet mask that differentiates between the network address and the host address. The section "Using Subnet-Zero to Get Around the Rules" later in this chapter covers how you can use the bottom subnet.Ĭontinue reading here: Finding Subnet Information Given a Host Address and the MaskĬlassful subnetting involves dividing a classful network into subnets. This leaves the middle six subnets available for deployment, but the top and bottom subnets are wasted. The top subnet has all ones in the subnet field and the bottom subnet contains all zeros. In traditional subnetting, you are not allowed to use the so-called top and bottom subnets. Table 1 -2 The Ei%ht Subnets for Example J-l The three bits that make up the subnet field are printed in boldface to emphasize the distinction between the subnet bits and the host bits. The eight subnets for Example 1-1 are listed in Table 1 -2. This yields eight unique combinations that are used to identify the subnets: 000.001.010,011, 100. A separator, such as a hyphen, makes it easier to read eight bits in a row.Įxample 1 1 uses three bits for the subnet field. Also, some people prefer one notation over the other, so a good idea is to be familiar with both.Īs you can see from Example 1 -1, converting from dotted-decimal notation to binary when subnetting is often convenient. For example, configuring an address on a router interface requires the mask in dotted decimal notation, but the output of show ip route favors slash notation in most versions of IOS. NOTE Both the dotted decimal and slash notations are acceptable, and both notations are used when working with Cisco routers.
The common way to write a major net together with its subnet mask is by using the shorthand notation of the major net followed by a slash (/) and the number of ones in the mask. Subnet mask in dotted decimal notation: 255.255.255.224 Length of original host field: 8 bits (from Figure 1-1) Number of host bits to steal for subnet field: 3 bits Number of host bits remaining after subnetting: 8-3= 5 bits Consider the following example that subnets major net 192.168.1.0 by stealing three bits from the host field to make a three-bit subnet field as shown in Example 1-1.Įxample 1-1 Subnetting a Class C. The best way to get familiar with subnetting is to practice.
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