For example, if a packet's destination IP address was and there
were three network prefixes in the routing table (,, and, the router would select the route to The 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 subnet. If the address is assigned
to a host that is attached to the or subnet, the
routing system will never route traffic to the host since the  longest
match algorithm  assumes that the host is part of the 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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