What is Subnetting | FLSM | VLSM Definitions Class C, FLSM, Mask 2, Nbsp, Subnet Mask, subnetting FacebookTwitter We use subnetting for reduce wastage of IP. There to Two Types of Subnetting. 1) FLSM = Fix Length Subnet Mask 2) VLSM =Variable subnet mask Tip : For Learn Sub-netting start from Class C. Its very easy , if you do practice daily.