CCNA Exam Prep Question of the Week

You have been assigned the 172.17.8.0/21 address space for your office. You have a few LANs with a maximum of 50 device addresses, and some point-to-point WANs. What mask should you use for the LANs and for the WANs?

  1. LANs: /26; WANs: /30
  2. LANs and WANs: /26
  3. LANs: /27; WANs: /30
  4. LANs: /28; WANs: /30
  5. LANs and WANs: /30

The correct answer is 1.

For 50 device addresses, 6 host bits are needed so a mask of /26 should be used for the LANs. On point-to-point WANs there are only 2 device addresses, so a mask of /30 should be used.

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Related Courses:
ICND1    —    Interconnecting Cisco Network Devices 1
ICND2    —    Interconnecting Cisco Network Devices 2
CCNAX    —    CCNA Boot Camp v1.1

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3 comments

  1. Thomas Reply

    I dont understand why the correct answer is not number 3. /27 and /30.

    /21 = 255.255.248.0 or 11111111.11111111.11111000.00000000.

    50 devices need to borrow 6 additional host bits which would give us a mask of /27.

    111111111.11111111.11111111.11100000.

    am I wrong here?

  2. Diane Teare Reply

    You are correct that the 50 devices need 6 host bits. However, it’s the subnet bits, not the host bits that are borrowed. The original address space is /21, which has 11 host bits. Since the 50 devices need 6 host bits, then there are 5 bits left for subnetting. Thus, /21 plus 5 bits for subnets results in a mask of /26 for the LANs:
    11111111.11111111.11111SSS.SSHHHHHH
    where “S” are subnet bits and “H” are host bits

  3. middleton Reply

    The questions and their answers given in such a format are really very helpful in cracking such exams.