Route aggregation with VLSM

Route aggregation with VLSM

1.1.5 This page will explain the benefits of route aggregation with VLSM.
When VLSM is used, it is important to keep the subnetwork numbers grouped together in the network to allow for aggregation. For example, networks like 172.16.14.0 and 172.16.15.0 should be near one another so that the routers only carry a route for 172.16.14.0/23. 
The use of classless interdomain routing (CIDR) and VLSM prevents address waste and promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997. 
Figure illustrates how route summarization reduces the burden on upstream routers. This complex hierarchy of variable-sized networks and subnetworks is summarized at various points with a prefix address, until the entire network is advertised as a single aggregate route of 200.199.48.0/20. Route summarization, or supernetting, is only possible if the routers of a network use a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry a prefix that consists of a 32-bit IP address and bit mask in the routing updates. In Figure , the summary route that eventually reaches the provider contains a 20-bit prefix common to all of the addresses in the organization. That address is 200.199.48.0/22 or 11001000.11000111.0011. For summarization to work, addresses should be carefully assigned in a hierarchical fashion so that summarized addresses will share the same high-order bits.
The following are important rules to remember:

• A router must know in detail the subnet numbers attached to it.
• A router does not need to inform other routers about each subnet if the router can send one aggregate route for a set of routes.
• A router that uses aggregate routes has fewer entries in its routing table.

VLSM increases route summarization flexibility because it uses the higher-order bits shared on the left, even if the networks are not contiguous. 
Figure shows that the addresses share the first 20 bits. These bits are colored red. The 21^st bit is not the same for all the routes. Therefore the prefix for the summary route will be 20 bits long. This is used to calculate the network number of the summary route.
Figure shows that the addresses share the first 21 bits. These bits are colored red. The 22^nd bit is not the same for all the routes. Therefore the prefix for the summary route will be 21 bits long. This is used to calculate the network number of the summary route.
The next page will teach students how to configure VLSM.

Calculating 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.

When to use VLSM

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.

A waste of space

A waste of space 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.

A waste of space

A waste of space
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.

A waste of space

A waste of space
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

VLSM

What is VLSM and why is it used?

 1.1.1

Certification-level claim: Compute and use Variable Length Subnet Masking (VLSM) techniques to design and implement effective and efficient IP addressing.
This module provides essential background information for the CCNA exam. Namely, this is how to configure IP addresses, subnet masks and gateway addresses on routers and hosts, and how to design an IP addressing scheme to meet design requirements.
Hands-on skills: None
In this lesson students will be introduced to the new topic of Variable Length Subnet Masks (VLSM). It is important for instructors to introduce this topic after they have made sure that students are thoroughly familiar with subnetting. It might be useful to give students the opportunity to demonstrate their skills at subnetting by giving them a series of small network addressing problems. These could be such as ones they have done in CCNA 1 and 2. Instructors should then emphasize that VLSM is an important topic and students will now be able to use subnet zero. During this module, try to give the students plenty of opportunities to compute and use VLSM techniques to design and implement effective and efficient IP addressing.
Best practices for teaching this TI include online study with study guides, group work, practical addressing quizzes using VLSM, lab work, and mini-lecture.
This is a core TI.
VLSM is simply an extension of basic subnetting, where the same Class A, B, or C address is subnetted by using masks of different lengths. VLSM provides a more efficient way of assigning IP addresses. It provides more flexibility in assigning an adequate number of hosts and subnets given a limited number of IP addresses. In CCNA 1 and 2, the question may have come up as to why host addresses are used on a WAN link, which only requires one address on either end of the link, plus a network address and a broadcast address. VLSM makes it possible to subnet a subnet so VLSM can be used on WAN links with a Classless InterDomain Routing (CIDR) notation of /30. IP subnet zero is enabled by default on Cisco IOS 12.0 and higher. This allows the use of all zeros and all ones subnets.
Pay particular attention to the following figures:

• Figure outlines that VLSM works with OSPF, IS-IS, EIGRP, RIP v2, and static routing.
• Figure emphasizes the use of the /30 on the serial links.
• Figures and illustrate VLSM and how it is computed.

The following are questions for the students to research:

1. Why is VLSM described as subnetting a subnet?

Why was VLSM not used in CCNA 1 and 2? 
=====================================

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.