Cisco CCNA

Using VLSM for Address Space Efficiency An Example

Suppose Widget, Inc., asks you to subnet one of its class C major nets and tells you it needs the following:

• Two subnets that can support at least 60 hosts

• Four subnets that can support at least 10 hosts

• As many subnets as possible that can support two hosts

The subnets are needed to support some new additions to its network, as summarized in Table 1-3.

Table 1 -3 Subnets Needed by Widget, Inc.

Subnet Size	Quantity Needed	Purpose
6()+ hosts	2	Branch offices
!()+ hosts	4	Server farms
2 hosts	As many us possible (use the remaining space)	Point-to-point home offices

First, you should do a quick check of the quantity of addresses needed. The branch offices require at least 120 host addresses (60 addresses times 2 branch offices), and the server farms require at least 40 host addresses (10 addresses times 4 farms). Any remaining addresses will be used for the point-to-point home offices, but this is not a hard requirement, so the basic need is for 160 (120 plus 40) addresses. This seems to be a reasonable request, because a class C has an 8-bit host field (see Figure 1-1), and an 8-bit host field with no subnetting can support up to 254 addresses (see "Calculating the Number of Host Addresses in a Subnet" earlier in this chapter). At least Widget, Inc., is not asking for the impossible; for example, it is not asking you to support 500 addresses with a single class C.

Next, tackle the largest subnets—the subnets for the branch offices. To accommodate the branch offices, you need to subnet the class C address space into chunks of at least 60 host addresses each This is done in the following section and represents an initial round of subnetting.

Round 1 of Subnetting

To start, you create four subnets that can support 62 hosts each. You can accomplish this by applying a 26-bit subnet mask to Widget's class C. Two of the resulting subnets will be deployed for branch offices, and the other two will be subnetted further to accommodate the other requirements. The following is Widget's class C and mask (the last octet of the mask is expanded into binary to help illustrate what's happening):

Widget, lnc.'s Major Net: 192.168.1.0 (8-bit host field)

Mask for round 1: 255.255.255.1100-0000 (/26 mask that supports 62 hosts per subnet)

The two bits printed in boldface represent the bits that were stolen to make a 2-bit subnet field.

Table 1-4 lists the subnets created by the first round of subnetting. The two bits that make up the subnet field are printed in boldface to emphasize the distinction between the subnet bits and the host bits.

Table 1 -4 Subnets Created by the Mask for Round J

Subnet Number in Subnet Number in

Name Binary (Last Octet) Decimal Proposed Use

Table 1 -4 Subnets Created by the Mask for Round J

Subnet Number in Subnet Number in

Name Binary (Last Octet) Decimal Proposed Use

Subnet 1	192.168.1.0000-0000	192.168.1.0/26	Subnet further; see round 2
Subnet 2	192.168.1.0100-0000	192.168.1.64/26	Branch Office A
Subnet 3	192.168.1 .IfMXMXXX)	192.168.1.128/26	Branch Office B
Subnet 4	192.168.1.1100-0000	192.168.1.192/26	Subnet further; see round 2

This first round of subnetting is nothing new—it's the same as traditional subnetting covered in "Subnetting a Glassful Address Space" earlier in this chapter. Stealing two bits for the subnet field leaves six bits in the host field and yields 2(\ or 64 combinations. Subtracting the two reserved addresses for the subnet and broadcast address leaves 62 addresses for hosts. This meets Widget, Inc.'s requirement for two subnets of at least 60 hosts, so set aside Subnet 2 and Subnet 3 for the two branch offices—they are ready for deployment. Subnet 2 and Subnet 3 are selected because they are middle subnets rather than top or bottom subnets (see "The Rules on Top and Bottom Subnets" earlier in this chapter).

Figure 1-4 depicts the subnets that are set aside and unused after round 1.

Figure 1-4 Widget, Inc. 's Address Space After Round I of Subnetting

Address space of major net 192.168.1.0

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