114 APPENDIX D: ADVANCED IP ROUTING CONCEPTS
maintenance and the risk of creating erroneous addresses unless you plan
the addressing scheme properly.
Guidelines for Using VLSMs
Consider the following guidelines when you implement VLSMs:
■ When you design the subnetwork scheme for your network, do not
estimate the number of subnetworks and hosts that you need. Work
from the top down until you are sure that you have accounted for all
the hosts, present and future, that you need.
■ Make sure that the routers forward routes based on what is known as
the longest match.
For example, assume that the destination IP address of a packet is
158.101.26.48 and that the following four routes are in the routing
table:
■ 158.101.26.0/24
■ 158.101.3.10/16
■ 158.101.26.32/16
■ 158.95.80.0/8
The router selects the route to 158.101.26.0/24 because its extended
network prefix has the greatest number of bits that correspond to the
destination IP address of the packet.
See RFCs 1219 and 1878 for information about understanding and using
VLSMs.
Supernetting Because Class B Internet addresses are in short supply, larger networks
are now usually granted a contiguous block of several Class C addresses.
Unfortunately, this creates very large routing tables since multiple Class C
routes have to be defined for each network containing more than 254
nodes. Larger routing tables mean more work for the routers and,
therefore, poorer performance.
Supernetting is only supported by RIPv2.
With traditional IP, each class C network must have a routing table entry.
Supernetting, or CIDR (Classless InterDomain Routing), is a technique
that allows each of these larger networks to be represented by a single
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