Explaining the Need for VLSM and CIDR

VLSM is defined as the capability to specify a different subnet mask for the same network number on different subnets. VLSM can help optimize available address space.

CIDR is a technique that is supported by BGP-4 and based on route aggregation. CIDR enables routers to group routes to reduce the quantity of routing information that is carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity.

32 Chapter 1: Networking and Routing Fundamentals

Why are VLSM and CIDR needed? This answer is this: IP address depletion. This means that the current IP address scheme, which is known as IPv4, is beginning to run out of IP addresses. This is an unacceptable situation that many network engineers deal with every day. CIDR and VLSM are only interim solutions but are nonetheless effective. CIDR was needed because too many specific network numbers were filling the Internet routing table a few years ago. CIDR was invented to solve this problem and save the Internet from collapse.

NOTE Not only is address depletion an issue, but also many networks are faced with large routing tables that need to be reduced in size to enable smoother network and router operation.

When this addressing scheme was first designed many years ago, the engineers most likely believed that it would be sufficient. Nevertheless, the recent explosive growth of the Internet and corporate intranets has made new technology and strategies necessary to deal with this looming problem. The situation becomes even more critical when you consider that corporations of all sizes are beginning to use the Internet as a means of revenue. It's an exciting time for our field when you consider that less than fifteen years ago, computers and networks were things of ponderous size with only specific applications. This is a time of constant change and advancement, and it is interesting to consider what the world of technology will be like for the next generation.

One of the most interesting enhancements on the horizon is IP version 6 (IPv6), also known as IP next generation (IPng), which is in its developmental stage. This is a move to improve the existing IPv4 implementation, which is quickly reaching critical mass. The proposal was released in July 1992 at the Boston Internet Engineering Task Force (IETF) meeting. IPv6 tackles issues such as the IP address depletion problem, quality of service capabilities, address autoconfiguration, authentication, and security capabilities.

NOTE IPv6 is shipping today in Cisco IOS Software Release 12.2 T. Visit the following website:


Because these issues are facing us in the here and now, it is in response to these concerns that the technologies of VLSM and CIDR were developed. Not only do these techniques enable us to better use the remaining IP addresses, but also they have enabled large networks to continue growing without the routers becoming saturated by the various routes within the network. A prime example of this is the Internet. This example will be discussed in further detail, but keep it in mind as you read through these sections.

Several items used within the discussions of VLSM and CIDR are important to discuss before preceding any further.

Explaining the Need for VLSM and CIDR 33

Route Summarization

Route summarization, also known as aggregation or supernetting, is a method of representing a series of network numbers in a single summary address that is advertised to other routers. For example, assume that a router knows about the following networks that are attached to it:

The router would summarize that information to other routers by saying "I know how to get to these networks in the summarization" Subnetting essentially extends the prefix to the right by making the router know a specific IP addresses; summarization, on the other hand, reduces the prefix to the left, thereby enabling the router to advertise only the higherorder bits. Figure 1-12 demonstrates how subnetting and route summarization differ.

Figure 1-12 Comparison of Subnetting and Route Summarization



Length increased

Subnetting increases the prefix size to enable very specific routes.

Prefix Host -Length decreased

Route summarization decreases the prefix size to enable very general or summarized rates.

TIP Refer to Chapter 3 of Cisco's "Internetwork Design Guide" for an overview that describes the best practices concerning summarization. You can also visit the following website: www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/

An example of route summarization is CIDR, which is discussed in detail later in this chapter. The following requirements enable route summarization to work properly:

• Multiple IP addresses must share the same high-order bit to be properly summarized.

• Routing tables and protocols must base their routing decisions on a normal 32-bit IP address and a variable prefix length.

• Routing protocols must carry this prefix (such as /16, which translates into a mask) with the 32-bit IP address in the routing table.

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• Remember, not all routing protocols can carry a subnet mask; those that don't are referred to as classful routing protocols. The following sections discuss classful and classless routing protocols.

Classful Routing

Classful routing always summarizes routes by the major network numbers. RIP and Interior Gateway Routing Protocol (IGRP) are protocols that use this type of routing. They are called classful because they always consider the network class. This automatic summarization is always done at network boundaries.

Impact of Classful Routing

The use of classful routing has some considerable impact on a network. For one thing, subnets are not advertised to a different major network. In addition, noncontiguous subnets are not visible to each other. Figure 1-13 illustrates how classful routing and subnetting can affect your network.

Certain techniques have been developed to assist in overcoming this problem: IP unnumbered, secondary addressing, and using OSPF. Further discussion of classful routing and the issues surrounding its use (that is, discontiguous subnets) are beyond the scope of this book.

Figure 1-13 How Classful Routing and Subnetting Affect the Network

Each router has a subnet that it attaches to, but in a classful environment, they cannot and will not be advertised because the subnets are not on a classful boundary

Each router has a subnet that it attaches to, but in a classful environment, they cannot and will not be advertised because the subnets are not on a classful boundary

Classless Routing

Classless routing differs from classful routing in that the prefix length is transmitted. This prefix length is evaluated at each place it is encountered throughout the network. In other words, it can be changed to advertise routes differently at different locations within a network. This capability of classless routing enables more efficient use of IP address space and reduces routing traffic. A good example of this type of routing is OSPF. Classless routing has the following characteristics:

• One routing entry might match a block of host, subnet, or network addresses.

• Routing tables can be much shorter.

• Switching performance can be faster unless Cisco express forwarding (CEF) is used.

• Routing protocol traffic is reduced.

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  • Rhea
    How is vlsm and cidr supported on ospf?
    4 years ago
  • miia
    Why do we use cidr and vslm on the same network?
    1 year ago
  • eemeli
    Why you need to use to both vlsm and cidr on the same network?
    10 months ago
  • faryl
    Is route summarization and CIDR the dame?
    9 months ago

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