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Embed code for: 09.8 Implementing VLSM
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09.8 Implementing VLSM
9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/9/pages/8 1/4 Implementing VLSM Section 9: IP Addressing and Subnets Implementing VLSM involves further subnetting an already subnetted network address. In fact, it is often referred to as "subnetting a subnet." For example, assume that network 172.16.0.0 has already been subnetted by applying a 20bit subnet mask. One of the resulting subnet addresses, 172.16.32.0/20, is used for the region of the enterprise network that is shown in the following figure. This region needs to assign addresses to multiple LANs. Each LAN must have 50 hosts. You can use VLSM to further subnet address 172.16.32.0/20 to give you more subnet addresses with fewer hosts per subnet. The next figure shows in binary the original subnetting of the 172.16.0.0/16 network by borrowing four host bits, which provided 16 subnets with 4094 host addresses each. The figure also shows how further subnetting with VLSM increases the number of subnets and provides the desired number of host addresses per subnet. Counting the additional host bits that are borrowed by powers of two from left to right shows that 64 subnets are created. Counting the host bits by powers of two from right to left and then subtracting two shows that 62 host addresses are available per subnet. 9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/9/pages/8 2/4 The next figure shows the subnet addresses and host addresses that are achieved by using VLSM. The subnet for the region in this example, subnet 172.16.32.0/20, is further subnetted by applying a 26bit mask as shown in the previous figure. 9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/9/pages/8 3/4 The following figure shows some of the new VLSM subnet addresses applied to the regional network. To calculate the subnet addresses for the WAN links, further subnet one of the unused /26 subnets with a 30bit subnet mask. For this example, subnet 172.16.33.0 will be further subnetted. The 30bit subnet mask provides 16 (2 ) subnets with two 2 (2 – 2) host addresses each. 4 2 9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/9/pages/8 4/4 As seen in this example, the easiest way to assign the subnets is to assign the largest first. Up Next: Challenge