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Variable Length Subnet Masks (VLSMs)

133

FIGURE 3.3

Fourteen subnets with VLSM applied

By using a VSLM design, we save address space!

192.168.1.32/28

192.168.1.48/28

192.168.1.96/28

192.168.1.112/28

F0/0

F0/1

192.168.1.64/28 F0/0

192.168.1.80/28 F0/1

F0/0

F0/1

Lab D

Lab E

Lab F

_

_

_

S0/1

S0/1

S0/1

S0/0

192.168.1.4/30

S0/0

Lab_A F0/0

192.168.1.16/30

Lab_B

S0/1

F0/0

192.168.1.20/30

192.168.1.128/28

192.168.1.8/30

S0/0

192.168.1.144/28

192.168.1.12/30

S0/0

Lab_C F0/0

192.168.1.160/28

Why bother with VLSM design?

You have just been hired to a new company and need to add onto the existing network. There is no problem with starting over with a new IP address scheme. Should you use a VLSM class- less network or a classful network?

Let’s just say you happen to have plenty of address space because you are using the Class A 10.0.0.0 private network address in your corporate environment and can’t even come close to imagining that you’d ever run out of IP addresses. Why would you want to bother with the VLSM design process?

Good question. There’s a good answer too!

Because by creating contiguous blocks of addresses to specific areas of your network, you can then easily summarize your network and keep route updates with a routing protocol to a min- imum. Why would anyone want to advertise hundreds of networks between buildings, when you can just send one summary route between buildings and achieve the same result?

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