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Cisco CCNA (200-301)Network FundamentalsMedium

A network administrator receives documentation stating a subnet mask of /21 for a newly assigned network. Which subnet mask in dotted-decimal notation corresponds to this CIDR value?

  1. A255.255.248.0
  2. B255.255.240.0
  3. C255.255.255.0
  4. D255.255.252.0
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Correct answer: A. 255.255.248.0

A /21 mask means 21 bits are network bits. The first three octets (24 bits) are all 1s, and the fourth octet has 24 total minus... actually the third octet needs 5 bits from it: 21 = 8+8+5, so the third octet is 11111000 = 248. Thus the mask is 255.255.248.0.

Why the other options are wrong

  • B. 255.255.240.0 corresponds to /20, not /21.
  • C. 255.255.255.0 corresponds to /24, a much smaller network with more host bits removed.
  • D. 255.255.252.0 corresponds to /22, one bit fewer network bits.

CIDR to Subnet Mask Conversion

Converting a CIDR prefix length to dotted-decimal notation by counting network bits set to 1 across the four octets.

  • /24 = 255.255.255.0
  • /21 = 255.255.248.0 (21 bits = 8+8+5)
  • Third octet value comes from 256 minus 2^(8-remaining bits)

Memory trick: 'Count the ones, fill the octets' — 8+8+5=21.

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