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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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