Implementing VLSM, Medium-Sized Routed Network Construction

Subnetting Review

To identify subnets, you will “borrow” bits from the host ID portion of the IP address:

  • The number of subnets available depends on the number of bits borrowed.

–The available number of subnets = 2s, I which s is the number of bits borrowed.

  • The number of hosts per subnet available depends upon the number of host ID bits not borrowed.

–The available number of hosts per subnet = 2h -2, in which h is the number of host bits not borrowed.

–One address is reserved as the network address.

–One address is reserved as the broadcast address.

Possible Subnets and Hosts for a Class C Network


Possible Subnets and Hosts for a Class B Network


Possible Subnets and Hosts for a Class A Network


Subnetting Review Exercise

Subnet a network with a private network address of so that it provides 100 subnets and maximizes the number of host addresses for each subnet.

  • How many bits will need to be borrowed?
  • What is the new subnet mask?
  • What are the first four subnets?
  • What are the range of host addresses for the four subnets?

What Is a Variable-Length Subnet Mask?



Subnet is divided into smaller subnets.

–Subnet with one mask (/27).

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

A Working VLSM Example


A Working VLSM Example (Cont.)


A Working VLSM Example (Cont.)


A Working VLSM Example (Cont.)



Understanding Route Summarization


Routing protocols can summarize addresses of several networks into one address.

Classful Routing Overview

  • Classful routing protocols do not include the subnet mask with the network in the routing advertisement.
  • Within the same network, consistency of the subnet masks is assumed, one subnet mask for the entire network.
  • Summary routes are exchanged between foreign networks.
  • Examples of classful routing protocols include:



Note: Classful routing protocols are legacy routing protocols typically used to address compatibility issues. RIP version 1 and Interior Gateway Routing Protocol (IGRP) are introduced to provide examples.

Classless Routing Overview

  • Classless routing protocols include the subnet mask with the network in the advertisement.
  • Classless routing protocols support VLSM; one network can have multiple masks.
  • Summary routes must be manually controlled within the network.
  • Examples of classless routing protocols include:




  • RIPv2 and EIGRP act classful by default, and summary routes are exchanged between foreign networks.

–The no auto-summary command forces these protocols to behave as if they are classless.

Summarizing Within an Octet




Route Summarization Operation in Cisco Routers



  • Supports host-specific routes, blocks of networks,
    and default routes
  • Routers use longest prefix match

Summarizing Routes in a Discontiguous Network


  • Classful RIPv1 and IGRP do not advertise subnets, and therefore cannot support discontiguous subnets.
  • Classless OSPF, EIGRP, and RIPv2 can advertise subnets, and therefore can support discontiguous subnets.



– Subnetting lets you efficiently allocate addresses by taking one large broadcast domain and breaking it up into smaller more manageable broadcast domains.
– VLSMs let you more efficiently allocate IP addresses by adding multiple layers of the addressing hierarchy.
– The benefits of route summarization include smaller routing tables and the ability to isolate topology changes.

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