Kiến thức về công nghệTCP IP VLSM

When to use VLSM?... Calculating subnets with VLSM... Exercise Calculating subnets with VLSM Using VLSMs, define appropriate subnets for addressing the networks using 192.168.10.0/24..

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VLSM

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Issues with IP Addressing

• IP addressing crisis

• As early as 1992, the IETF identified two specific

concerns:

– IP address exhaustion

– Routing table growth

U N I V E R S I T Y

Internet

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IP Addressing Solutions

– Subneting masking: RFCs 950, 1812

– V ariable- l ength s ubnet m asks: RFC 1812

– C lassless i nter d omain r outing: RFCs 1518, 1519, 2050

– Address allocation for private Internets: RFC

1918

– N etwork A ddress T ranslation: RFC 1631

– Route summarization : RFC 1518

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172.16.14 64/27

172.16.14.96/27

What is VLSM ?

• Subnet 172.16.14.0/24 is divided into smaller subnets

(Sub-subnet):

– Subnet with one mask at first (/27)

– Further subnet one of these subnets not used elsewhere (/30)

C B

A

HQ

172.16.1.0/24

172.16.2.0/24

HQ 172.16.0.0/16

172.16.14.136/30 172.16.14.132/30

172.16.14.140/30

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A waste of space

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When to use VLSM?

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Calculating subnets with VLSM

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Calculating subnets with VLSM (2)

Derived from the

172.16.32.0/20 Subnet

172.16.33.0/30 172.16.32.0/26

172.16.32.64/26

172.16.32.128/26

172.16.32.192/26 26-Bit Mask (62 Hosts)

Derived from the

172.16.33.0/26 Subnet

30-Bit Mask (2 Hosts)

172.16.33.12/30 172.16.33.8/30 172.16.33.4/30

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Exercise Calculating subnets with

VLSM

Using VLSMs, define appropriate subnets for

addressing the networks using 192.168.10.0/24.

28 Hosts KL

Sydney

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Exercise Calculating subnets with

VLSM

60 Hosts 12 Hosts 12 Hosts

192.168.10.0/24

KL

Sydney

192.168.10.64/27

192.168.10.128/30

192.168.10.132/30 192.168.10.136/30

28 Hosts

192.168.10.0/26 192.168.10.96/28 192.168.10.112/28

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Route aggregation with VLSM

KL Sydney

Perth

Singapore

192.168.48.0/24

192.168.49.0/24

192.168.50.0/24

192.168.51.0/24

192.168.52.0/24

192.168.53.0/24

192.168.54.0/24

192.168.55.0/24

192.168.56.0/24

192.168.57.0/24

2.1 68

.56 .0/

21 192.168.52.0/22

192.168.48.0/20

192 16 8.4 8.0 /22

Upstream provider

Route summarization reduces routing table size by aggregating routes to multiple networks into one supernet

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Route summarization example

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