Variable Length Subnet Masks (VLSM) are used to create subnetworks of varying sizes. This can be done as long as the IP address spaces of the subnets do not overlap. VLSM gives network designers the ability to not waste public IPv4 addresses by creating networks in sizes they need. Early dynamic routing protocols were not designed to work with VLSM because they were designed around classful IPv4 addressing. Modern routing protocols are designed to work with VLSM and classless inter-domain routing (CIDR). For the Cisco CCNA exam, you will need to know how to create subnets of varying sizes that do not have overlapping address spaces.
In the video tutorials below, I demonstrate how to solve a typical variable length subnet mask multiple choice question. The type of which you might see on an exam.
Video Tutorials – VLSM