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What Is VLSM and Why Is It Used?

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Classful and Classless Routing

Classful (basic subnetting)

• Classful routing protocols require that a single network use the same subnet mask

• RIP v1 required that all connected networks have the same subnet mask

• So couldn’t subnet a subnet.

Classless (uses VLSM)

• VLSM allows a single autonomous system to have networks with different subnet masks.

• Often referred to as "subnetting a subnet"

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Support for Classless Routing

Open Shortest Path First (OSPF)

• Integrated Intermediate System to Intermediate System (Integrated IS-IS)

• Enhanced Interior Gateway Routing Protocol (EIGRP)

• RIP v2

• static routing

• (but not RIP v1, IGRP, EGP)

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A Waste of Space

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Subnet Zero

• Not recommended by CISCO with normal subnetting

Due confusion when a network and a subnet have the same addresses

• OK to use subnet zero with VLSM

Cisco IOS v12.0 routers use subnet zero by default

Or use the “no ip subnet-zero” command

• Not recommended by CISCO to use last subnet

Due confusion b/w a network broadcast and a subnet directed broadcast

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When to Use VLSM?

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Wasting Host Addresses on WAN Connections

• Consider class C address 192.168.187.0/27 network

3 bits for subnet = 8 – 1 = 7 useable subnets

5 bits for hosts = 30 hosts per subnet

• But we don’t want to waste 3 x 30 host subnets on the WAN connections so we subnet one of the subnets

• Must therefore have VLSM enabled routers

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Using VLSM for WAN Connections

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Subnetting Details

Subnet 6 (/27) further subnetted with mask of /30 to provide 8 subnets in total with 4 hosts per subnet (2 bits).

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Further Example

• Could use 255.255.255.0 subnet mask - 172.16.0.0/24

255 networks with 255 hosts per network

But we would waste 253 hosts for WAN

• Similar problem to version 2 TCS using RIP V1

• So use /24 subnet mask and further subnet one subnet with a /30 mask for the WAN

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Further Example

• For example, consider the subnet address 172.16.32.0/20 and a network needing ten host addresses.

• With this subnet address, there are over 4000 (212 – 2 = 4094) host addresses, most of which will be wasted.

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VLSM Solution

• Given 172.16.32.0/20

• Gives 16 subnets with 4096 hosts per subnet

Wasteful

• Further subnet one of the sixteen subnets into a /26

• giving 64 subnets with 64 hosts

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Subnet Details

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A Working VLSM Example

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Classless InterDomain Routing (CIDR) and VLSM

• Can only subnet a subnet that is empty

• Classless InterDomain Routing (CIDR) and VLSM not only prevents address waste, but also promotes route aggregation (OR summarization).

• Classless routing protocols carry a prefix that consists of 32-bit IP address and bit mask in the routing updates

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Configuring VLSM

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RIP History

• RIPv1 has the following limitations:

It does not send subnet mask information in its updates.

It sends updates as broadcasts on 255.255.255.255.

It does not support authentication.

It is not able to support VLSM or classless interdomain routing (CIDR).

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RIPv2 Features

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Comparing RIPv1 and RIPv2

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Configuring RIPv2