Content Overview A network administrator must anticipate and manage the physical growth of a network, perhaps by buying or leasing another floor of the building to house new networking equipment such as racks, patch panels, switches, and routers. The network designer must choose an addressing scheme that allows for growth. Variable-Length Subnet Masking (VLSM) is a technique that allows for the creation of efficient, scalable addressing schemes. With the phenomenal growth of the Internet and TCP/IP, virtually every enterprise must now implement an IP addressing scheme. Many organizations select TCP/IP as the only routed protocol to run on their network. Unfortunately, the architects of TCP/IP could not have predicted that their protocol would eventually sustain a global network of information, commerce, and entertainment. Twenty years ago, IP version 4 (IPv4) offered an addressing strategy that, although scalable for a time, resulted in an inefficient allocation of addresses. IP version 6 (IPv6), with virtually unlimited address space, is slowly being implemented in select networks and may replace IPv4 as the dominant protocol of the Internet. 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 in order to meet the changing needs of users. 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 does much to determine the scalability of the network. Therefore, it is important that the routing protocol be chosen wisely. Routing Information Protocol (RIP) is still considered suitable for small networks, but is not scalable to large networks because of inherent limitations. To overcome these limitations yet maintain the simplicity of RIP version 1 (RIP v1), RIP version 2 (RIP v2) was developed. Students completing this module should be able to:
Content 1.1 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. One technique is called Variable-Length Subnet Masks (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 use VLSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF), Integrated Intermediate System to Intermediate System (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. Implementing VLSM is often referred to as "subnetting a subnet", and can be used to maximize addressing efficiency. Classful routing protocols require that a single network use the same subnet mask. Therefore, network 192.168.187.0 must use just one subnet mask such as 255.255.255.0. VLSM is simply a feature that allows a single autonomous system to have networks with different subnet masks. If a routing protocol allows VLSM, use a 30-bit subnet mask on network connections, 255.255.255.252, a 24-bit mask for user networks, 255.255.255.0, or even a 22-bit mask, 255.255.252.0, for networks with up to 1000 users.
Content 1.1 VLSM 1.1.2 A waste of space In the past, it has been recommended that the first and last subnet not be used. Use of the first subnet, known as subnet zero, for host addressing was discouraged because of the confusion that can occur when a network and a subnet have the same addresses. The same was true with the use of the last subnet, known as the all-ones subnet. It has always been true that these subnets could be used. However, it was not a recommended practice. As networking technologies have evolved, and IP address depletion has become of real concern, it has become acceptable practice to use the first and last subnets in a subnetted network in conjunction with VLSM. In this network, the network management team has decided to borrow three bits from the host portion of the Class C address that has been selected for this addressing scheme. If management decides to use subnet zero, it has eight useable subnets. Each may support 30 hosts. If the management decides to use the no ip subnet-zero command, it has seven usable subnets with 30 hosts in each subnet. From Cisco IOS version 12.0, remember that Cisco routers use subnet zero by default. Therefore 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 three remaining subnets for the WAN links, it will have used all of the available addresses and have 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. Using this addressing scheme one third of the potential address space will have been wasted. Such an addressing scheme is fine for a small LAN. However, this addressing scheme is extremely wasteful if using point-to-point connections.
Content 1.1 VLSM 1.1.3 When to use VLSM? It is important to design an addressing scheme that allows for growth and does not involve wasting addresses. This section examines how VLSM can be used to prevent waste of addresses on point-to-point links.This time the networking team decided to avoid their wasteful use of the /27 mask on the point-to-point links. The team decided to apply VLSM to the addressing problem. To apply VLSM to the addressing problem, the team will break the Class C address into subnets of variable sizes. Large subnets are created for addressing 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. In this case this is the best solution for the point-to-point connections. The team will take one of the three subnets they had 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.
Content 1.1 VLSM 1.1.4 Calculating subnets with VLSM VLSM helps to manage IP addresses. VLSM allows for the setting of a subnet mask that suits the link or the 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. Look at the example in Figure which illustrates how to calculate subnets with VLSM. 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 are using a classful routing protocol the WAN link would need to be a subnet of the same Class B network, assuming that the administrator is not using IP unnumbered. Classful routing protocols such as RIP v1, IGRP, and EGP are not capable of supporting VLSM. Without VLSM, the WAN link would have to have the same subnet mask as the LAN segments. A 24-bit mask (255.255.255.0) would support 250 hosts. The WAN link only needs two addresses, one for each