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VLSM (Variable Length Subnet Masking) help better use of IP addresses space. We can assign to LAN and WAN segments without any waste using Variable Length Subnet Mask (VLSM). As shown in the scenario in Figure below, the hosts in each of the sub-network will be assigned a subnet according to the number of the host to decrease the waste of IP addresses.

The first host IPv4 address for each subnet is assigned to the LAN interface of the router. The WAN interfaces of the routers are assigned the IP addresses and mask for the /30 subnets. Hosts on each subnet will have a host IPv4 address from the range of host addresses for that subnet and an appropriate mask. Hosts will use the address of the attached router LAN interface as the default gateway address. The table below is the addressing scheme for the above mention network scenario.

In the previous lesson, we have assigns the one subnet to each network. Each network contains 126 usable hosts. You can see that there are a lot of IPs are going waste in-network human resource, quality control and between the WAN ports of the routers. So, we can ignore the waste of IP addresses using VLSM subnetting.

So, first of all, we can assign the first three subnet of the previous lesson to management, finance and sales department because the number of the host in these LANs are between 64 and 128, so we need a network with /25 prefix which can accommodate 126 hosts.

We can further subnet the Net-3 into /26, /27, and 8x /30 networks. The network with /26 prefix can accommodate up to 62 host, and the network with /27 prefix can accommodate up to 30 hosts while the network with /30 prefix is generally used for point to point connectivity. The figure below illustrates the assigned IP addresses to the scenario which we have already discussed in the previous lesson, but you can see that now we have saved many of IP addresses for future use.


VLSM Chart

An addressing chart is very important for identifying the blocks of addresses already used and the block available for future use as shown in the table below. The VLSM chart helps the network administrator to avoid assigning addresses that have previously been allocated.

Net-0 Assign to Management
Net-1 Assign to Sales
Net-2 Assign to Finance
Net-3 Required further subnetting into /26 Prefix
Net-3a Assign to Human Resource
Net-3b Required further subnetting into /27 Prefix
Net-3b1 Assign to Quality Control
Net-3b2 Required further subnetting into /30 Prefix
Net-3b2A Assign to WAN Network between Routers
Net-3b2B Spare for future Expensionn
Net-3b2C Spare for future Expensionn
Net-3b2D Spare for future Expensionn
Net-3b2E Spare for future Expensionn
Net-3b2F Spare for future Expensionn
Net-3b2G Spare for future Expensionn
Net-3b2H Spare for future Expensionn
Net-4 Spare for future Expensionn
Net-5 Spare for future Expensionn
Net-6 Spare for future Expensionn
Nett-7 Spare for future Expensionn


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