Variable-Length Subnet Masking (VLSM)
Variable-Length Subnet Masking or VLSM amounts to “subnetting subnets”, meaning that VLSM allows network administrators to divide an IP address space into a hierarchy of subnets of different sizes, making it possible to create subnets with different host counts wasting smaller numbers of IP addresses.
In VLSM a subnet mask defines the size of the subnet (the amount of host addresses in the subnet). Fixed-Length Subnet Masking (FLSM) creates the same size subnets where some of these subnets will have many hosts and some have few. This results in some subnets having many unused addresses, or some sets of hosts being too large to fit into a subnet. Where Variable-Length Subnet Masking is enabled, a large subnet can be divided into a set of smaller “sub-subnets”, which can be used to handle smaller sets of hosts.
For instance, consider a Class C address space like 192.168.2.0 and an organization with four departments: the accounting with 75 devices; the students with 50 devices; the HR with 25 devices; and the library with 20 devices. Under fixed subnetting, dividing the 255 host addresses available into four subnets would support only 62 hosts each, not meeting the needs of the accunting department and vastly oversupplying addresses for HR and library.
Using Variable-Length Subnet Masking (VLSM), the space is first split in 2, with each subnet able to address 126 hosts. One subnet covers accounting. The other is split in two, supplying two sub-subnets of 62 hosts. One covers the students, the other is split in two once more, creating two 30-host sub-sub-subnets, to cover HR and library.
In order to use Variable-Length Subnet Masking, a network administrator must use a routing protocol that supports it, such as Open Shortest Path First (OSPF), routing Information Protocol v2 (RIPv2), Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediate System-to-Intermediate System (IS-IS) and Border Gateway Protocol (BGP).
Variable-Length Subnet Masking is similar in concept and intent to Classless Inter Domain Routing (CIDR), which allows a single Internet domain to have an address space that does not fit into traditional IP address classes. Variable-Length Subnet Masking was originally defined in IETF RFC 1812.