defined as
unallocated. With VLSM, IP address space can be used more efficiently. As an example, let's
use a conventional classful address to configure a serial line and then compare it to an address
that uses VLSM.
When the network 172.1.100.1/30 is used, the address space between 5 and 255 remains
available for other networks. In this example, more than 250 addresses are conserved using
VLSM! Distance vector routing protocols such as RIP and IGRP do not advertise classless
prefixes in their routing protocol updates.
classes and allows for better scalability when supporting the advertising of IP prefixes. CIDR
allows routers to group routes to cut down on the quantity of routing information carried by the
core routers. With CIDR, several IP networks appear to networks outside the group as a single,
larger entity. CIDR completely changes the concept of address classes. It does not categorize a
network by class. Instead, routes are viewed as a combination of addresses and masks. As long
as the network has a common prefix, it doesn't matter where the boundary exists between
network and host.
routing table, a router announces a single network that shows the summarized prefix to its
neighbor routers. When traffic is forwarded to the source router, the routers can switch packets
to the appropriate subnet by applying the longest match rule. Summarization collapses the
prefix to the left, and subnetting extends the prefix to the right. Figure 5-5 depicts this idea.