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Chapter 1
Concepts and terminology
A prefix is expressed either in dotted decimal notation, for example {10.0.0.0; 255.255.255.0} or in prefix/
length notation, for example10.0.0.0/24.
Longest Prefix Matching
Finding the best matching destination for a given destination IP address can be reformulated in finding the
most specific prefix amongst the matching prefixes. The best matching prefix is synonymous with longest
prefix. In practice this is realised by executing the
A possible way to implement LPM is to sort the FIB entries from most specific to least specific. If for a given
destination an entry is found in a sequential search through the table, this will automatically be the best
matching entry, having the longest prefix.
Of course alternative FIB search strategies exist to accomplish LPM.
The picture below shows an example of Longest Prefix Matching.
Packet with destination address 192.168.1.20 is sent to interface 2, as 192.168.1.x is the longest prefix
matching packet's destination address. For the same reason the packet with destination address 24.168.1.12
is sent to interface 1.
E-DOC-CTC-20051017-0168 v1.0
longest prefix matching
Destination network
24.168.1.x
192.168.1.x
192.168.x.x
...
192.168.1.20
24.168.1.12
(LPM) algorithm.
Intf
1
2
3
...
1
2
7
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