Implementing Variable-Length Subnet Masks
Variable-length subnet masks (VLSM) were developed to enable multiple levels of subnetworked
IP addresses within a single network. This strategy can be used only when it is supported by the
routing protocol in use, such as Routing Information Protocol version 2 (RIPv2), OSPF, and
EIGRP. VLSM is a key technology on large, routed networks. Understanding its capabilities is
important when planning large networks.
Reviewing Subnets
Prior to working with VLSM, it is important to have a ?¬?rm grasp on IP subnetting. When you are
creating subnets, you must determine the optimal number of subnets and hosts.
Computing Usable Subnetworks and Hosts
Remember that an IP address has 32 bits and comprises two parts: a network ID and a host ID.
The length of the network ID and host ID depends on the class of the IP address. The number of
hosts available also depends on the class of the IP address.
The default number of bits in the network ID is referred to as the classful pre?¬?x length. Therefore,
a Class C address has a classful pre?¬?x length of /24, a Class B address has a classful pre?¬?x length
of /16, and a Class A address has a classful pre?¬?x length of /8.
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