What subnet mask is represented by the slash notation /20?

Question

BlackTom AI · Accepted Answer

A /20 prefix means the subnet mask has 20 consecutive 1 bits from the left.

1) Relevant knowledge points:
- IPv4 masks are 32-bit values expressed as a sequence of 1s (network) followed by 0s (hosts).
- Dotted-decimal masks break the 32 bits into four 8-bit octets; each octet value is the decimal equivalent of its 8-bit binary.
- Common octet binary-to-decimal conversions: 11111111=255, 11110000=240, 11100000=224, 11111000=248, 00000000=0.

2) Step-by-step reasoning:
- /20 means 20 ones. Allocate these ones across octets left-to-right.
- First octet: 8 ones -> 11111111 -> 255.
- Second octet: next 8 ones -> 11111111 -> 255.
- Remaining ones: 20 - 16 = 4 ones go into the third octet -> 11110000.
- Fourth octet: no ones remain -> 00000000.

3) Intermediate calculations:
- Third octet binary: 11110000
  - 128 + 64 + 32 + 16 + 0 + 0 + 0 + 0 = 240
- So full mask in dotted decimal: 255.255.240.0
- Additional check: host bits = 12 (32 - 20), addresses per subnet = 2^12 = 4096, usable hosts typically = 4096 - 2 = 4094 (if using traditional network/broadcast reservation).

4) Why the correct answer is correct:
- The binary distribution of 20 ones produces third-octet value 240, giving 255.255.240.0 exactly matching /20.

5) Why other options are incorrect:
- a. 255.255.255.0 is /24 (24 ones), not /20.
- c. 255.255.224.0 is /19 (third octet 11100000 -> 224), not /20.
- d. 255.255.255.248 is /29 (29 ones, last octet 248), not /20.

6) Conclusion:
The correct subnet mask for /20 is 255.255.240.0, which matches option b.

What subnet mask is represented by the slash notation /20?单项选择题

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