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This section discusses VLSMs and how they can be used to further maximize IPv4 addressing efficiency.

Variable-Length Subnet Masks

VLSM allows an organization to use more than one subnet mask within the same network address space. Implementing VLSM is often called subnetting a subnet. It can be used to maximize addressing efficiency.

Consider Table 2-5, in which the subnets are created by borrowing 3 bits from the host portion of the Class C address,

Table 2-5 Subnetting with One Mask

Subnet Number

Subnet Address

Subnet 0

Subnet 1

Subnet 2

Subnet 3

Subnet 4

Subnet 5

Subnet 6

Subnet 7

If the ip subnet-zero command is used, this mask creates seven usable subnets of 30 hosts each. Four of these subnets can be used to address remote offices at Sites A, B, C, and D, as shown in Figure 2-8.

Figure 8Figure 2-8 Using Subnets to Address a WAN


Unfortunately, only three subnets are left for future growth, and three point-to-point WAN links between the four sites remain to be addressed. If the three remaining subnets were assigned to the WAN links, the supply of IP addresses would be completely exhausted. This addressing scheme would also waste more than a third of the available address space.

There are ways to avoid this kind of waste. Over the past 20 years, network engineers have developed three critical strategies for efficiently addressing point-to-point WAN links:

  • Use VLSM

  • Use private addressing (RFC 1918)

  • Use IP unnumbered

Private addresses and IP unnumbered are discussed in detail later in this chapter. This section focuses on VLSM. When VLSM is applied to an addressing problem, it breaks the address into groups or subnets of various sizes. Large subnets are created for addressing LANs, and very small subnets are created for WAN links and other special cases.

A 30-bit mask is used to create subnets with two valid host addresses. This is the exact number needed for a point-to-point connection. Figure 2-9 shows what happens if one of the three remaining subnets is subnetted again, using a 30-bit mask.

Figure 9Figure 2-9 Subnetting with VLSMs


Subnetting the subnet in this way supplies another eight ranges of addresses to be used for point-to-point networks. For example, in Figure 2-10, the network can be used to address the point-to-point serial link between the Site A router and the Site B router.

Figure 10Figure 2-10 Using VLSM to Address Point-to-Point Links


Example 2-1 shows the commands needed to configure the Site A router, labeled RTA, with a 27-bit mask on its Ethernet port and a 30-bit mask on its serial port.

Example 2-1 Configuring VLSM

RTA(config)#interface e0
RTA(config-if)#ip address
RTA(config-if)#interface s0
RTA(config-if)#ip address

Interactive Media Activity Drag and Drop: VLSM Calculation

After completing this activity, you will have a better understanding of VLSM.

Lab 2.10.1 Configuring VLSM and IP Unnumbered

In this lab, you will configure VLSM and test its functionality with two different routing protocols, RIPv1 and RIPv2. Finally, you will use IP unnumbered in place of VLSM to further conserve addresses.

Classless and Classful Routing Protocols

For routers in a variably subnetted network to properly update each other, they must send masks in their routing updates. Without subnet information in the routing updates, routers would have nothing but the address class and their own subnet mask to go on. Only routing protocols that ignore the rules of address class and use classless prefixes work properly with VLSM. Table 2-6 lists common classful and classless routing protocols.

Table 2-6 Classful and Classless Routing Protocols

Classful Routing Protocols

Classless Routing Protocols

RIP Version 1

RIP Version 2









Routing Information Protocol version 1 (RIPv1) and Interior Gateway Routing Protocol (IGRP), common interior gateway protocols, cannot support VLSM because they do not send subnet information in their updates. Upon receiving an update packet, these classful routing protocols use one of the following methods to determine an address's network prefix:

  • If the router receives information about a network, and if the receiving interface belongs to that same network, but on a different subnet, the router applies the subnet mask that is configured on the receiving interface.

  • If the router receives information about a network address that is not the same as the one configured on the receiving interface, it applies the default, subnet mask (by class).

Despite its limitations, RIP is a very popular routing protocol and is supported by virtually all IP routers. RIP's popularity stems from its simplicity and universal compatibility. However, the first version of RIP, RIPv1, suffers from several critical deficiencies:

  • RIPv1 does not send subnet mask information in its updates. Without subnet information, VLSM and CIDR cannot be supported.

  • RIPv1 broadcasts its updates, increasing network traffic.

  • RIPv1 does not support authentication.

In 1988, RFC 1058 prescribed the new and improved Routing Information Protocol version 2 (RIPv2) to address these deficiencies. RIPv2 has the following features:

  • RIPv2 sends subnet information and, therefore, supports VLSM and CIDR.

  • RIPv2 multicasts routing updates using the Class D address, providing better efficiency.

  • RIPv2 provides for authentication in its updates.

Because of these key features, RIPv2 should always be preferred over RIPv1, unless some legacy device on the network does not support it.

When RIP is first enabled on a Cisco router, the router listens for version 1 and 2 updates but sends only version 1. To take advantage of the RIPv2 features, turn off version 1 support, and enable version 2 updates with the following commands:

Router(config)#router rip
Router(config-router)#version 2

The straightforward RIP design ensures that it will continue to survive. A new version has already been designed to support future IPv6 networks.

Lab 2.10.2a VLSM 1

In this lab, you create an addressing scheme using VLSM.

Lab 2.10.2b VLSM 2

In this lab, you create an addressing scheme using VLSM.

Lab 2.10.2c VLSM 3

In this lab, you create an addressing scheme using VLSM.

Lab 2.10.2d VLSM 4

In this lab, you create an addressing scheme using VLSM.

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