With fixed-length subnet masking (FLSM), a similar number of addresses is allocated for each subnet. It is a sequence of numbers of unchanging length that streamlines packet routing within the subnets of a proprietary network. If all the subnetworks have similar requirements for the number of hosts, these fixed-size address blocks would be enough.
But that is most frequently not the case. Fixed-length subnet masking (FLSM) is also referred to as conventional subnetting. The traditional subnetting method wastes IP addresses because the same number of addresses is allocated to each subnetwork even though the requirements are not similar.
The topology shown in Figure 1 above requires 5 subnets, one for the four LANs and one for WAN connection between routers. Using traditional subnetting with the address of 130.10.0.0/23, we can borrow a bit from the third octet and 2 bits from the last octet of the host portion to meet the subnet need of 5 subnets.
Though traditional subnetting meets the requirements of the largest LAN and divides the address space into enough subnets, it results in the major waste of unused addresses.
For example, only two addresses are required for a WAN subnet. However, each subnet has 62 usable addresses, and 60 unused addresses are available in these subnets. This also limits the network’s growth by reducing the total number of subnets available.
This incompetent use of addresses is a feature of traditional subnetting. Traditional subnetting schemes in this scenario are not professional and are full of waste. To avoid a waste of IP address subnetting a subnet, or using a Variable Length Subnet Mask (VLSM), was designed. Figure 3 shows the pie chart for the above fixed-length subnet masking (FLSM) table.
Applying a traditional subnetting scheme to this scenario is inefficient and wasteful. This example is a good model for showing how to use subnetting a subnet to maximize address utilization. Subnetting a subnet, or using a variable-length subnet mask (VLSM), to avoid wasting addresses.
