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What is Route summarization?

Combining the routes of several consecutive networks into a single route with a shorter prefix. For example, the four networks 10.0.4.0/24 to 10.0.7.0/24 summarize to 10.0.4.0/22. It keeps routing tables small and avoids passing small internal changes to the outside, but if the summarized range includes addresses that do not exist, it also draws in packets that have nowhere to go.

Japanese name: 経路集約

Route summarization appears in 1 practice question across 1 topic, including Network Design and Operations.

Field: Network Design and Operations

Try it: questions about Route summarization

Each of the following three sets consists of four consecutive /24 networks. For which set can the four routes be summarized into exactly one /22 route without including any addresses outside the set?

  1. A10.0.2.0/24 to 10.0.5.0/24
  2. B10.0.4.0/24 to 10.0.7.0/24
  3. C10.0.6.0/24 to 10.0.9.0/24
Show answer and explanation

Answer: B. 10.0.4.0/24 to 10.0.7.0/24

In a /22, the lowest two bits of the third octet fall in the host part, so one /22 consists of four consecutive /24s whose third octet starts at a multiple of 4. For 10.0.4.0/24 to 10.0.7.0/24, the third octet runs from 4 to 7 (00000100 to 00000111 in binary) and the upper 22 bits are common, so they summarize to 10.0.4.0/22. 10.0.2.0/24 to 10.0.5.0/24 straddles 10.0.0.0/22 and 10.0.4.0/22, and 10.0.6.0/24 to 10.0.9.0/24 straddles 10.0.4.0/22 and 10.0.8.0/22, so neither forms a single /22. Summarizing routes (route summarization) keeps routing tables small, but including networks that do not exist pulls in packets that have nowhere to go.

Route summarization in the explanations

Questions that mention Route summarization (1)

  1. Each of the following three sets consists of four consecutive /24 networks. For which set can the four routes be summarized into exactly one /22 route without including any addresses outside the set?

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Related terms

IP routing Packet ACL (access control list) Administrative distance BGP (Border Gateway Protocol) Broadcast domain Link aggregation Longest prefix match OSPF (Open Shortest Path First) Path MTU discovery

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