When to Use VLSM

1.1.3 

It is important to design an address scheme that allows for growth and does not waste addresses. This page examines how VLSM can be used to prevent the waste of addresses on point-to-point links.

As shown in Figure , the network management team has decided to avoid the wasteful use of the /27 mask on the point-to-point links. The team applies VLSM to the address problem.

To apply VLSM to the address problem, the team breaks the Class C address into subnets of variable sizes. Large subnets are created for LANs. Very small subnets are created for WAN links and other special cases. A 30-bit mask is used to create subnets with only two valid host addresses. This is the best solution for the point-to-point connections. The team will take one of the three subnets they previously decided to assign to the WAN links, and subnet it again with a 30-bit mask.

In the example, the team has taken one of the last three subnets, subnet 6, and subnetted it again. This time the team uses a 30-bit mask. Figures and illustrate that after using VLSM, the team has eight ranges of addresses to be used for the point-to-point links.

The next page will teach students how to calculate subnets with VLSM.

Calculating subnets with VLSM

1.1.4

VLSM helps to manage IP addresses. This page will explain how to use VLSM to set subnet masks that fit the link or segment requirements. A subnet mask should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another.

The example in Figure shows a network that requires an address scheme.

The example contains a Class B address of 172.16.0.0 and two LANs that require at least 250 hosts each. If the routers use a classful routing protocol, the WAN link must be a subnet of the same Class B network. Classful routing protocols such as RIP v1, IGRP, and EGP do not support VLSM. Without VLSM, the WAN link would need the same subnet mask as the LAN segments. A 24-bit mask of 255.255.255.0 can support 250 hosts.  

The WAN link only needs two addresses, one for each router. That means that 252 addresses would be wasted.

If VLSM was used, a 24-bit mask would still be applied on the LAN segments for the 250 hosts. A 30-bit mask could be used for the WAN link because only two host addresses are needed.

Figure shows where the subnet addresses can be applied based on the number of host requirements. The WAN links use subnet addresses with a prefix of /30. This prefix allows for only two host addresses which is just enough for a point-to-point connection between a pair of routers.

In Figure , the subnet addresses used are generated when the 172.16.32.0/20 subnet is divided into /26 subnets.

To calculate the subnet addresses used on the WAN links, further subnet one of the unused /26 subnets. In this example, 172.16.33.0/26 is further subnetted with a prefix of /30. This provides four more subnet bits and therefore 16 (2⁴) subnets for the WANs. Figure illustrates how to work through a VLSM system.

VLSM can be used to subnet an already subnetted address. For example, consider the subnet address 172.16.32.0/20 and a network that needs ten host addresses. With this subnet address, there are 2¹² – 2, or 4094 host addresses, most of which will be wasted. With VLSM it is possible to subnet 172.16.32.0/20 to create more network addresses with fewer hosts per network. When 172.16.32.0/20 is subnetted to 172.16.32.0/26, there is a gain of 2⁶, or 64 subnets. Each subnet can support 2⁶ – 2, or 62 hosts.

Use the following steps to apply VLSM to 172.16.32.0/20:

1. Write 172.16.32.0 in binary form.
2. Draw a vertical line between the 20^th and 21^st bits, as shown in Figure . The original subnet boundary was /20.
3. Draw a vertical line between the 26^th and 27^th bits, as shown in Figure . The original /20 subnet boundary is extended six bits to the right, which becomes /26.
4. Calculate the 64 subnet addresses with the bits between the two vertical lines, from lowest to highest in value. The figure shows the first five subnets available.

It is important to remember that only unused subnets can be further subnetted. If any address from a subnet is used, that subnet cannot be further subnetted. In Figure , four subnet numbers are used on the LANs. The unused 172.16.33.0/26 subnet is further subnetted for use on the WAN links.

The Lab Activity will help students calculate VLSM subnets.

The next page will describe route aggregation.

A Waste of VLSM

1.1.2 This page will explain how certain address schemes can waste address space.

In the past, the first and last subnet were not supposed to be used. The use of the first subnet, which was known as subnet zero, was discouraged because of the confusion that could occur if a network and a subnet had the same address. This also applied to the use of the last subnet, which was known as the all-ones subnet. With the evolution of network technologies and IP address depletion, the use of the first and last subnets have become an acceptable practice in conjunction with VLSM.

In Figure , the network management team has borrowed three bits from the host portion of the Class C address that has been selected for this address scheme.

If the team decides to use subnet zero, there will be eight useable subnets. Each subnet can support 30 hosts. If the team decides to use the no ip subnet-zero command, there will be seven usable subnets with 30 hosts in each subnet. Cisco routers with Cisco IOS version 12.0 or later, use subnet zero by default.

In Figure , the Sydney, Brisbane, Perth, and Melbourne remote offices may each have 30 hosts. The team realizes that it has to address the three point-to-point WAN links between Sydney, Brisbane, Perth, and Melbourne. If the team uses the last three subnets for the WAN links, all of the available addresses will be used and there will be no room for growth. The team will also have wasted the 28 host addresses from each subnet to simply address three point-to-point networks. This address scheme would waste one-third of the potential address space.

Such an address scheme is fine for a small LAN. However, it is extremely wasteful if point-to-point connections are used.

The next page will explain how VLSM can be used to prevent wasted addresses.

 

VLSM

1.1.1  What is VLSM and why is it used?

As IP subnets have grown, administrators have looked for ways to use their address space more efficiently. This page introduces a technique called VLSM. With VLSM, a network administrator can use a long mask on networks with few hosts, and a short mask on subnets with many hosts. -

In order to implement VLSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF), Integrated IS-IS, Enhanced Interior Gateway Routing Protocol (EIGRP), RIP v2, and static routing.

VLSM allows an organization to use more than one subnet mask within the same network address space. VLSM implementation maximizes address efficiency, and is often referred to as subnetting a subnet.

Classful routing protocols require that a single network use the same subnet mask. As an example, a network with an address of 192.168.187.0 can use just one subnet mask, such as 255.255.255.0.

A routing protocol that allows VLSM gives the network administrator freedom to use different subnet masks for networks within a single autonomous system.  Figure shows an example of how a network administrator can use a 30-bit mask for network connections, a 24-bit mask for user networks, and even a 22-bit mask for networks with up to 1000 users.

The next page will discuss network address schemes.

Semester 3 

Module 1: Introduction to Classless Routing - Overview

Network administrators must anticipate and manage the physical growth of networks. This may require them to buy or lease another floor of a building for new network equipment such as racks, patch panels, switches, and routers. Network designers must choose address schemes that allow for growth. Variable-length subnet mask (VLSM) is used to create efficient and scalable address schemes.

Almost every enterprise must implement an IP address scheme. Many organizations select TCP/IP as the only routed protocol to run on their networks. Unfortunately, the architects of TCP/IP did not predict that the protocol would eventually sustain a global network of information, commerce, and entertainment.

IPv4 offered an address strategy that was scalable for a time before it resulted in an inefficient allocation of addresses. IPv4 may soon be replaced with IP version 6 (IPv6) as the dominant protocol of the Internet. IPv6 has virtually unlimited address space and implementation has begun in some networks. Over the past two decades, engineers have successfully modified IPv4 so that it can survive the exponential growth of the Internet. VLSM is one of the modifications that has helped to bridge the gap between IPv4 and IPv6.

Networks must be scalable since the needs of users evolve. When a network is scalable it is able to grow in a logical, efficient, and cost-effective way. The routing protocol used in a network helps determine the scalability of the network. It is important to choose the routing protocol wisely. Routing Information Protocol version 1 (RIP v1) is suitable for small networks. However, it is not scalable to large networks. RIP version 2 (RIP v2) was developed to overcome these limitations.

This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.  

Students who complete this module should be able to perform the following tasks:

• Define VLSM and briefly describe the reasons for its use
• Divide a major network into subnets of different sizes using VLSM
• Define route aggregation and summarization as they relate to VLSM
• Configure a router using VLSM
• Identify the key features of RIP v1 and RIP v2
• Identify the important differences between RIP v1 and RIP v2
• Configure RIP v2
• Verify and troubleshoot RIP v2 operation

Configure default routes using the ip route and ip default-network commands