The basic concept of VLSMs is to provide more flexibility by dividing a network into multiple subnets. The trick to using this technique is ensuring that you have an adequate number of hosts allocated per subnet.

NOTE Not every protocol supports VLSM. If you decide to implement VLSM, make sure that you are using a VLSM-capable routing protocol, such as OSPF, BGP, Enhanced IGRP (EIGRP), Intermediate System-to-Intermediate System (IS-IS), and RIP Version 2 (RIP-2).

OSPF and static routes support VLSMs. With VLSMs, you can use different masks for the same network number on different interfaces, which enables you to conserve IP addresses for better efficiency. VLSMs do this by allowing both big subnets and small subnets. As previously mentioned, you need to ensure that the number of hosts is sufficient for your needs within each subnet.

In Example 1-1, a 30-bit subnet mask is used, leaving 2 bits of address space reserved for serial line host addresses. There is sufficient host address space for two host endpoints on a point-to-point serial link.

Example 1-1 VLSM Demonstration interface ethernet 0

ip address

! 8 bits of host address space reserved for Ethernet hosts interface serial 0

ip address

! 2 bits of address space reserved for serial lines

! System is configured for OSPF and assigned 107 as the process number router ospf 107

! Specifies network directly connected to the system network area

As Example 1-1 demonstrates, VLSM is efficient when used on serial lines because each line requires a distinct subnet number, even though each line has two host addresses. This requirement wastes subnet numbers. However, if you use VLSM to address serial links in a core router, you can save space. In Figure 1-14, the regular subnet is further subnetted with 6 additional bits. These additional subnets make 63 additional subnets available. VLSM also enables the routes within the core to be summarized as

36 Chapter 1: Networking and Routing Fundamentals

Figure 1-14 VLSM Conserves Subnets

Most early networks never had their IP addresses assigned to them in a way that would enable network engineers to group them in blocks. Instead, they had been assigned as needed, so massive renumbering projects would need to be performed—not one of the most popular pastimes of anyone involved in networking. However, although hindsight is 20/20, remember the past when considering the future and newer technology, such as IPv6. Otherwise, you might end up doing quite a lot of static routing and odd configuring just to keep your network stable.

VLSM Design Guidelines and Techniques

To assist you when designing the use of VLSM within your network, consider the following guidelines:

• Optimal summarization occurs with contiguous blocks of addresses.

• If small subnets are grouped, routing information can be summarized.

• Group VLSM subnets so that routing information can be consolidated.

• Allocate VLSM by taking one regular subnet and subnetting it further.

• Avoid using two different classful subnet masks inside a given network address.

In conclusion, you might ask yourself why there are questions about implementing VLSM. As previously mentioned, VLSM is not supported by every protocol, although it is

supported by OSPF, EIGRP, ISIS, and RIP-2. So these newer protocols might have to coexist with older protocols that do not support VLSM and would have trouble routing. In addition, the use of VLSM can be difficult. If it is not properly designed, it can cause the network to not operate properly, and it increases the complexity of troubleshooting any network.

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