VLSM (Variable Length Subnet Mask) is a technique that allows network administrators to divide an IP address space into subnets of different sizes, rather than dividing it into subnets of the same size. This allows for more efficient use of IP addresses, as smaller subnets can be used for smaller networks, and larger subnets can be used for larger networks.
For example, if a network administrator is given a Class C IP address space of 192.168.1.0/24, they can use VLSM to divide it into two subnets: one with a /25 mask for a smaller network, and one with a /26 mask for a larger network. The smaller network would have 128 host addresses, and the larger network would have 64 host addresses.
This allows the administrator to assign IP addresses more efficiently, as they can use the larger subnet for a network that needs more IP addresses and the smaller subnet for a network that needs fewer IP addresses.