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If in $\text{CIDR}$ notation an $\text{IP}$ address is written as $$, the subnet mask is:

  1. $$
  2. $$
  3. $$
  4. $$
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2 Answers

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 CIDR general notation format:$a.b.c.d/n$ where n is the number of bits that are presented in network id.

  • NID+SID= n bits
  • HID=(32-n) bits

Subnet mask:  it is $32$ bit number in which the number of 1’s indicates network id and subnet id, number of 0’s indicates host id part.

  • NID+SID=all 1’s
  • HID=all 0’s

Given IP address is $$, here /25 represents the first 25 bits are all 1’s. that is

$$ can be represented in binary form:

$11111111.11111111.11111111.10000000$, last $7$ 0’s represented as HID.

Therefore SM would be: $$

So option $(D)$ is correct.


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CIDR Representation:                                                /25 shows no of bit in Network Id


Subnet Mask=1 in Network Id Pat and 0 in Host Id Part.

Host ID part=32-25


i.e 25 bit Network Id + 7 bit Host Id

So Subnet Mask= 11111111.11111111.11111111.10000000


Hence,D is the correct Option.


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