© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-1 Determining IP Routes Implementing Variable-Length Subnet Masks

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-2 Outline Overview VLSM Benefits VLSM Calculations Route Summarization with VLSM Route Summarization Implementation Considerations Route Summarization Management Summary

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-3 What Is a Variable-Length Subnet Mask? Subnet /24 is divided into smaller subnets –Subnet with one mask (/27) –Then further subnet one of the unused /27 subnets into multiple /30 subnets

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-4 Calculating VLSMs

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-5 A Working VLSM Example

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-6 Routing protocols can summarize addresses of several networks into one address. What Is Route Summarization?

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-7 Summarizing Within an Octet

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-8 Summarizing Addresses in a VLSM-Designed Network

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-9 Implementation Considerations Multiple IP addresses must have the same highest-order bits. Routing decisions are made based on the entire address. Routing protocols must carry the prefix (subnet mask) length.

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-10 Route Summarization Operation in Cisco Routers Supports host-specific routes, blocks of networks, and default routes Routers use longest match /32 Host /27Subnet /24Network /16Block of Networks /0Default

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-11 Summarizing Routes in a Discontiguous Network RIPv1 and IGRP do not advertise subnets, and therefore cannot support discontiguous subnets. OSPF, EIGRP, and RIPv2 can advertise subnets, and therefore can support discontiguous subnets.

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-12 Summary VLSMs provide more efficient use of IP addresses and have greater capability to use route summarization. VLSMs can provide more network addresses and fewer hosts per network. Route summarization enables a router to summarize several addresses into a single network number. Route summarization reduces memory use on routers and routing protocol network traffic. Cisco routers manage route summarization by performing automatic summarization and by selecting routes from route summaries.

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-13