© 2002, Cisco Systems, Inc. All rights reserved.
© 2002, Cisco Systems, Inc. All rights reserved.
2
Variable-Length
Subnet Masks
Objectives
Upon completing this lesson, you will be able to:
Explain the operation of variable-length subnet masks on Cisco routers
What Is a Variable-Length
Subnet 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
Calculating VLSMs
A Working VLSM Example
What Is Route
Summarization?
Routing protocols can summarize addresses of several networks into one address
Summarizing Within an Octet
Summarizing Addresses in a VLSM-Designed Network
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.
Route Summarization Operation in Cisco Routers
Supports host-specific routes, blocks of networks, default routes
Routers use the 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 Networks
0.0.0.0 /0 Default
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.
Summary
When an IP network is assigned more than one subnet mask, it is considered a network with variable-length subnet masks, overcoming the limitation of a fixed number of fixed-size subnetworks imposed by a single subnet mask.
In large internetworks, hundreds or even thousands of network addresses can exist. In these environments, it is often not desirable for routers to maintain many routes in their routing table. Route summarization, also called route aggregation or supernetting, can reduce the number of routes that a router must maintain by representing a series of network numbers in a single summary address.