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Determining IP Routes

Implementing Variable-Length Subnet Masks

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Outline

Overview

VLSM Benefits

VLSM Calculations

Route Summarization with VLSM Implementation of Route Summarization

Route Summarization Management

Summary

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What Is a Variable-LengthSubnet Mask?

Subnet 172.16.14.0/24 is divided into smaller subnets

Subnet with one mask (/27)

Then further subnet one of the unused /27 subnets into multiple /30

subnets

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Calculating VLSMs

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A Working VLSM Example

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What Is Route Summarization?

Routing protocols can summarize addresses of severalnetworks into one address.

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Summarizing Within an Octet

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Summarizing Addresses in aVLSM-Designed Network

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Implementation Considerations

Multiple IP addresses must have the samehighest-order bits.

Routing decisions are made based on the

entire address.

Routing protocols must carry the prefix(subnet mask) length.

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Route Summarization Operationin Cisco Routers

Supports host-specific routes, blocks of networks,and default routes

Routers use longest match

192.16.5.33 /32 Host

192.16.5.32 /27 Subnet

192.16.5.0 /24 Network

192.16.0.0 /16 Block of Networks0.0.0.0 /0 Default

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Summarizing Routes in aDiscontiguous Network

RIPv1 and IGRP do not advertise subnets, and therefore cannotsupport discontiguous subnets.

OSPF, EIGRP, and RIPv2 can advertise subnets, and therefore cansupport discontiguous subnets.

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Summary

VLSMs provide more efficient use of IP addresses andhave greater capability to use route summarization.

VLSMs can provide more network addresses and fewerhosts per network.

Route summarization enables a router to summarizeseveral addresses into a single network number.

Route summarization reduces memory use on routers androuting protocol network traffic.

Cisco routers manage route summarization by performingautomatic summarization and by selecting routes fromroute summaries.

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