For example, if a packet's destination IP address was 11.1.2.5 and there

were three network prefixes in the routing table (11.1.2.0/24,

11.1.0.0/16, and 11.0.0.0/8), the router would select the route to

11.1.2.0/24. The 11.1.2.0/24 route would be selected because its prefix

has the greatest number of corresponding bits in the Destination IP

address of the packet. This concept is illustrated in Figure 20. 

F I G U R E   2 0 .   B e s t   M a t c h   R o u t e   w i t h   L o n g e s t   P re f i x   ( M o s t

S p e c i f i c )

A very subtle but extremely important issue is that since the destina 

tion address matches all three routes, it must be assigned to a host that

is attached to the 11.1.2.0/24 subnet. If the 11.1.2.5 address is assigned

to a host that is attached to the 11.1.0.0/16 or 11.0.0.0/8 subnet, the

routing system will never route traffic to the host since the  longest

match algorithm  assumes that the host is part of the 11.1.2.0/24 sub 

net. Great care must be taken when assigning host addresses to ensure

that every host is reachable. 

Topologically Significant Address Assignment

Since OSPF and I IS IS convey the extended network prefix information

with each route, the VLSM subnets can be scattered throughout an

organization's topology. However, to support hierarchical routing and

reduce the size of an organization's routing tables, addresses should be

assigned so that they are topologically significant. 

Hierarchical routing requires that addresses be assigned to reflect the

actual network topology. This reduces the amount of routing informa 

tion by aggregating the set of addresses assigned to a particular region

of the topology into a single routing advertisement for the entire set.

Hierarchical routing allows this to be done recursively at various points

within the hierarchy of the routing topology. If addresses do not have a

topological significance, they cannot be aggregated and the size of the

routing tables cannot be reduced. 

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