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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

If you’re confused about what summary routes are, let me explain. Summarization, also called supernetting, provides route updates in the most efficient way possible by adverting many routes in one advertisement instead of individually. This saves a ton of bandwidth and mini- mizes router processing. As always, you use blocks of addresses (remember that block sizes are used in all sorts of networks) to configure your summary routes and watch your network’s performance hum.

But know that summarization only works if you design your network carefully. If you carelessly hand out IP subnets to any location on the network, you’ll notice straight up that you no longer have any summary boundaries. And you won’t get very far with creating summary routes with- out those, so watch your step!

Prefix

Mask

Hosts

Block Size

/25

128

126

128

/26

192

62

64

/27

224

30

32

/28

240

14

16

/29

248

6

8

/30

252

2

4

Implementing VLSM Networks

To create VLSMs quickly and efficiently, you need to understand how block sizes and charts work together to create the VLSM masks. Table 3.5 shows you the block sizes used when cre- ating VLSMs with Class C networks. For example, if you need 25 hosts, then you’ll need a block size of 32. If you need 11 hosts, you’ll use a block size of 16. Need 40 hosts? Then you’ll need a block of 64. You cannot just make up block sizes—they’ve got to be the block sizes shown in Table 3.7. So memorize the block sizes in this table—it’s easy. They’re the same numbers we used with subnetting!

TABLE 3.7

Block Sizes

The next step is to create a VLSM table. Figure 3.4 shows you the table used in creating a VLSM network. The reason we use this table is so we don’t accidentally overlap networks.

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