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Network Design and Operations

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
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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.

  • A. The smallest route that covers all four is 10.0.0.0/21, not a /22, because 2 is not a multiple of 4.
  • B. Correct. They summarize exactly to 10.0.4.0/22.
  • C. The smallest route that covers all four is 10.0.0.0/20, not a /22, because 6 is not a multiple of 4.
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More Network Design and Operations questions

  1. 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?
  2. Branch router R2 is connected to head-office router R1 over a 10.255.0.0/30 link, with R1's end at 10.255.0.1 and R2's at 10.255.0.2. When a static route to the head-office network 192.168.1.0/24 is configured on R2, which address is appropriate as the next hop?
  3. A router has learned routes to the same destination, 192.168.50.0/24, from both OSPF and RIP. On this router, the administrative distance, which expresses the preference for each route source (lower is preferred), is 110 for OSPF and 120 for RIP. Which route goes into the routing table?
  4. A router's routing table holds three routes: a static route 10.0.0.0/8 (next hop router A, administrative distance 1), 10.1.0.0/16 learned by OSPF (router B, 110) and 10.1.1.0/24 learned by RIP (router C, 120). To which router is a packet for 10.1.1.5 sent?
  5. Which statement correctly describes a link-state routing protocol?

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