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Embed code for: 09.7 Benefits of VLSM
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09.7 Benefits of VLSM
9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/9/pages/7 1/2 Benefits of VLSM Section 9: IP Addressing and Subnets In all the subnetting examples in previous topics, the same subnet mask was applied for all the subnets. This meant that each subnet had the same number of available host addresses. You may need this in some cases, but, in most cases, having the same subnet mask for all subnets ends up wasting address space. For example, in the following figure, Class B network 172.16.0.0 is subnetted by borrowing eight host bits and applying a 24bit subnet mask; this allows for 256 subnets with 254 host addresses each. In this example, many host addresses are wasted. Each WAN link needs only two host addresses, so 252 host addresses are wasted on each WAN link. Many host addresses are also wasted on other subnets. VLSM provides a solution. VLSM allows you to include more than one subnet mask within a network to achieve more efficient use of IP addresses. Instead of using the same subnet mask for all subnets, you can use the most efficient subnet mask for each subnet. The most efficient subnet mask for a subnet is the mask that provides an appropriate number of host addresses for that individual subnet. For example, subnet 172.16.6.0 has only 19 hosts, so it does not need the 254 host addresses that the 24bit mask allows. A 27bit mask would provide 30 host addresses, which is much more appropriate for this subnet. In the next figure, the 172.16.0.0/16 network is again divided into subnetworks using a 24bit subnet mask. However, one of the subnetworks in this range, 172.16.14.0/24, is further divided into smaller subnetworks using a 27bit mask to accommodate the subnets that have 19 or 28 hosts. These smaller subnetworks range from 9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/9/pages/7 2/2 172.16.14.0/27 to 172.16.14.224/27. Then, one of these smaller subnets, 172.16.14.128/27, is further divided using a 30bit mask, which creates subnets with only two hosts to be used on the WAN links. The subnets with the 30bit mask range from 172.16.14.128/30 to 172.16.14.156/30. In addition to providing a solution to the problem of wasted IP addresses, VLSM has another important benefit: support for route summarization, which is also called route aggregation. The hierarchical addressing design of VLSM enables easier summarization of network addresses. Route summarization reduces the number of routes in routing tables by representing a range of network subnets in a single summary address. Smaller routing tables require less CPU time for routing lookups. In the figure above, subnet 172.16.14.0/24 describes all the addresses that are further subnets of 172.16.14.0, including those from subnet 172.16.14.0/27 to subnet 172.16.14.128/30. VLSM is an important technology in large routed networks. It can only be used in networks that run routing protocols that support VLSM. These protocols include RIPv2, OSPF, and EIGRP. Up Next: Implementing VLSM