A variable-length subnet mask is also known as a classless Internet Protocol (IP) address because it allows a network administrator to apply different subnet masks to the same class address space. VLSM was originally defined in IETF RFC 1812.
Variable-length subnetting can be thought of as "subnetting a subnet." A mask defines the size of the subnet (the number of host addresses in the subnet). VLSM allows network engineers to divide an IP address space into subnets of different sizes. For subnets that need fewer addresses, the network engineer can use a mask with fewer host bits and the subnet will have fewer host IP addresses. The flexibility that VLSM provides reduces the number of wasted IP addresses in each subnet.
VLSM facilitates efficient use of IP address space in networks whose subnets are not filled to capacity. In networks having many unassigned IP addresses, a VLSM can provide more efficient use of address space than the more common fixed-length subnet mask (FLSM) in which the sequence of numbers always has the same length. Variable-length subnet masking also makes it easier for system administrators to change the configuration of a network as an organization evolves.
The VLSM is usually a string of binary digits shown over the subnet number, telling the router which parts of the subnet number to look at. A binary "1" over a particular digit in the subnet number says "Pay attention to this digit." A "0" says "Ignore this digit." In order to use VLSM, a network administrator must use a routing protocol that supports it. VLSM supports Routing Informaion Protocol v2 (RIPv2), Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS), Enhanced Interior Gateway Routing Protocol (EIGRP) and Border Gateway Protocol (BGP).
Keith Barker provides a simple use case for how (and why) to use VLSM in this video tutorial.
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