What is Longest prefix match?

The rule by which a router, when several routes in its routing table match a destination IP address, chooses the route with the longest (most specific) prefix. For example, a destination that matches both 10.0.0.0/8 and 10.1.0.0/16 uses the /16 route. The default route (0.0.0.0/0), used when nothing else matches, is the shortest possible route, with a length of 0.

Japanese name: ロンゲストマッチ(最長一致)

Longest prefix match appears in 2 practice questions across 2 topics, including Computer Networking Basics, Network Design and Operations.

Field: Network Design and Operations

Try it: questions about Longest prefix match

A router's routing table has four routes: 0.0.0.0/0 (next hop router A), 10.0.0.0/8 (router B), 10.1.0.0/16 (router C) and 10.1.2.0/24 (router D). To which router is a packet for 10.1.5.9 sent?

ARouter B
BRouter D
CRouter C

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Answer: C. Router C

Of the routes that match the destination address, a router chooses the one with the longest (most specific) prefix. This is called longest prefix match. 10.1.5.9 matches 0.0.0.0/0, 10.0.0.0/8 and 10.1.0.0/16, but it is not in the 10.1.2.0/24 range (10.1.2.0 to 10.1.2.255). The longest of the matches is /16, so the packet goes to router C. 0.0.0.0/0 is the default route that matches every destination and is used only when no other route matches.

A router's routing table has four routes: 172.16.0.0/12 (next hop router B), 172.16.32.0/19 (router A), 172.16.48.0/20 (router C) and 0.0.0.0/0 (router D). To which router is a packet for 172.16.45.10 sent?

ARouter A
BRouter B
CRouter C

Show answer and explanation

Answer: A. Router A

A router chooses, among the routes that match the destination, the one with the longest prefix (longest prefix match). The third octet, 45, is 00101101 in binary. A /19 compares the upper 3 bits of the third octet, and 45 AND 224 = 32, so the address matches 172.16.32.0/19 (172.16.32.0 to 172.16.63.255). A /20 compares the upper 4 bits, and 45 AND 240 = 32, which differs from 48, so it does not match 172.16.48.0/20 (172.16.48.0 to 172.16.63.255). The address also matches 172.16.0.0/12 and the default route, but the longest match, the /19, is chosen and the packet goes to router A.

Longest prefix match in the explanations

This is called longest prefix match.
A router's routing table has four routes: 0.0.0.0/0 (next h…
A router chooses, among the routes that match the destination, the one with the longest prefix (longest prefix match).
A router's routing table has four routes: 172.16.0.0/12 (ne…

Questions that mention Longest prefix match (2)

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Comparisons that include this term

Static vs Dynamic Routing: Failover, Scale and When to Use EachStatic vs dynamic routing in one table: how routes are set, behavior on failure, router load, configuration e…

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