Longest prefix match

From Academic Kids

Longest prefix match refers to an algorithm used by routers in Internet Protocol (IP) networking to select an entry from a routing table.

Because each entry in a routing table may specify a network, one destination address may match more that one routing table entry. The most specific table entry — the one with the highest subnet mask — is called the longest prefix match. It is called this because it is also the entry where the largest number of leading address bits in the table entry match those of the destination address.

For example, consider this IPv4 routing table (CIDR notation is used):

192.168.0.0/16
192.168.20.16/28

When the address 192.168.20.19 needs to be looked up, both entries in the routing table "match". That is, both entries contain the looked up address. In this case, the longest prefix of the candidate routes is 192.168.20.16/28, since its subnet mask (/28) is higher than the other entry's mask (/16), making the route more specific.

Routing tables often contain a default route, which has the shortest possible prefix match, to fall back on.

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