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 th...
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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 ...
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 255.255.25...
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 virtu...