4.2 VLSM :
VLSM (Variable Length Subnet Masking) allows efficient use of IP addresses. Networks implemented with VLSM can be summarized more efficiently due to manual control. With a distance vector protocol such as RIP , only one subnet mask value can be used on a network, as subnet mask values are not sent in routing updates.Variable Length Subnet Masking or VLSM gives the router the ability to announce a series of addresses with a single routing table entry. The way that is does this, is it looks at the addresses and ascertains which of those addresses are similar out to a designated bit pattern, also known as "high-order bits". It then looks at the subnet mask accompanying the update and decides which series of addresses are considered matches.
Variable-Length Subnet Masking: Variable-length subnet masking (VLSM) means implementing more than one mask on the same major class of a network. It allows for a more efficient use of IP address space both in terms of hosts and subnets, and can be essential on a network that does not have an abundance of IP address space. The following routing protocols support VLSM:
EIGRP is an advanced routing protocol that combines many of the features of both link-state and distance-vector routing protocols, EIGRP's DUAL algorithm contains many features which make it more of a distance vector routing protocol than a link-state routing protocol.