Fixed-Length Subnet Masking (FLSM)

With fixed-length subnet masking (FLSM) the similar number of addresses has 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 sub-network has similar requirements for the number of hosts, these fixed size address blocks would be enough. But, most frequently that is not the case. FLSM also referred to as conventional subnetting. The Traditional Subnetting method causes wastes of IP Addresses. Because the same number of addresses allocated to each sub-network even the requirement is 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, we can borrow w bit from the third octet and 2 bits from the last octet of the host portion to meet the subnet need of 5 subnets. As shown in Figure 2, borrowing 3 bits creates 8 subnets and leaves 6 host bits with 62 usable hosts per subnet. This plan creates the desired subnets and meets the host necessity of the largest LAN.

Though this traditional subnetting meets the requirements of the largest LAN and divides the address space into enough number of subnets, So it results in the major waste of unused addresses.

For example, only two addresses have required for WAN subnet, But each subnet has 62 usable addresses, there are 60 unused addresses available in this subnets. This also limits the growth of the network by reducing the total number of subnets available. This incompetent use of addresses is the feature of traditional subnetting. Traditional subnetting scheme to this scenario is not professional and also a full of waste. To avoid a waste of IP address subnetting a subnet, or using Variable Length Subnet Mask (VLSM), was design. Figure 3 Show the pie chart for the above table of fixed length subnet masking

Applying a traditional subnetting scheme to this scenario is not very efficient and is wasteful. Actually, this example is a good model for showing how we can use subnetting a subnet to maximize address utilization. Subnetting a subnet, or using a variable-length subnet mask (VLSM), to avoid wasting addresses.