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192.168.10.0/24 is to be split into subnets for site A (100 hosts), site B (50 hosts) and site C (25 hosts). Starting with the site that needs the most hosts, each site is given the smallest subnet that holds its hosts, packed in from the lowest addresses. Which subnet is assigned to site C?

  1. A192.168.10.160/27
  2. B192.168.10.192/27
  3. C192.168.10.192/28
Show answer and explanation

Answer: B. 192.168.10.192/27

A subnet with h host bits holds 2^h − 2 hosts. Site A needs /25 because 2^7 − 2 = 126 ≥ 100, so it gets 192.168.10.0/25 (.0 to .127), and site B needs /26 because 2^6 − 2 = 62 ≥ 50, so it gets 192.168.10.128/26 (.128 to .191). Site C needs /27 because 2^5 − 2 = 30 ≥ 25, so it gets the next block, 192.168.10.192/27 (.192 to .223). Assigning subnets with different prefix lengths like this is called VLSM (variable-length subnet masking). The remaining 192.168.10.224/27 can be split further, for example into /30 subnets for links between routers.

  • A. This is the next block if site B were given a /27. A /27 holds only 30 hosts, so it cannot fit site B's 50.
  • B. Correct. After the /25 and /26, a /27 is assigned starting at 192.168.10.192.
  • C. A /28 holds only 2^4 − 2 = 14 hosts, so it cannot fit site C's 25.
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