Cisco CCNA

Using VLSM Techniques with Classless Addressing

Classless addressing doesn't stop there. You can also break up the space any way you choose by using the same techniques of VLSM. (It's now called variable length network prefixes; however, the term VLSM is still commonly used, semantics aside). Remember, there are no more subnets by the true definition of the word because there is no subnet field—only a network prefix and its prefix mask. This means there's 110 such thing as a top or bottom subnet, so in using the techniques of VLSM, you can use all of the possible network prefixes for deployment into the network.

Continuing with Example 1-2, you can use VLSM techniques to divide the 192.168.4.0/22 space into smaller blocks with a longer network prefix. Example 1-3 illustrates the results of applying a /26 mask to 192.168.4.0/22 (this could be the first round of VLSM, for example).

Example 1-3 Using VLSM to Divide Supernet J92. J68.4.0/22 into Smaller Address Works

Original prefix: 192.168.4.0/22

Original prefix in binary: 192.168.0000 - 0100.00

New mask:

255.255.1111-1111.11

Original prefix bits New prefix Result of new mask A bjts applied to the r~ ^ ^ original prefix: 192.168.0000-01 xx.xxhh-hhhh

Prefix bits Host bits

In Example 1 -3, each x represents a bit that can be used to create new network prefixes and each h represents a host bit.

The new prefix mask /26 yields four bits that divide the original /22 space into 16 (24) smaller blocks. Each of these smaller blocks contains 6 bits for host addresses (up to 62 hosts each). Table 1-10 lists the blocks created with the 126 mask. The four new prefix bits resulting from the VLSM operation are printed in boldface for clarity.

Table 1-10 Listing of the New Prefixes Created from Example 1-3

Network Prefix (Subnet); h	Network Prefix (Subnet) In Dotted Decimal
Represents Host Bits	Notation
192.168.0000-0100.00/i/i-/i/2/t/2	192.168.4.0/26
192.168.0000-01 OOMhh-hhhh	192.168.4.64/26
192.168.0000-0100.10hh-hhhh	192.168.4.128/26
192.168.0000-0100.1 lhh-hhhh	192.168.4.192/26

Table 1-10 Listing of the New Prefixes Created from Example 13 (Continued)

Network Prefix (Subnet); h	Network Prefix (Subnet) in Dotted Decimal
Represents Host Bits	Notation
192.168.0000 010!Mhh-hhhh	192.168.5.0/26
192.168.00000101 Mhh-hhhh	192.168.5.64/26
192.168.0000-0101. Wih-hhhh	192.168.5.128/26
...
192.168.0000-0111.1 Ihh-hhhh	192.168.7.192/26

These blocks are equal in meaning to subnets—you deploy them as you would subnets with the same VLSM strategies. Note that the new prefix mask crosses over the traditional class C boundary (the dot after the 24th bit) without concern. This again demonstrates the power and flexibility of classless addressing. Also note that during deployment, you will need to verify that all of your network devices (including hosts) support classless addressing.

NOTE The terms subnet and network prefix are often used interchangeably. Many people are familiar with subnet and prefer using it even if they arc routing with VLSM and classless routing protocols. This book uses the term subnet when it is more descriptive than network prefix. Just keep in mind the semantics for any situation in which you have to adhere to strict definitions.

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