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In a class C address, 3 bits are borrowed for subnetting of host id part. Total possible number of subnet masks are _________________.
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According to standard convention, there should be exactly 1 subnet mask.

It is a Class C network, therefore, 24 Most Significant bits are for Network ID part and Remaining 8 bits are for Host ID part. Now, in the question, 3 bits from Host ID part are taken for subnetting.

So Subnet Mask has to be 11111111.11111111.11111111.11100000

But, it is not mentioned as to which 3 bits of the 8 bits (Host ID part) are used for subnetting. So Mathematically, we have $^{8}C_{3}$ choices to make and thus $^{8}C_{3}$ = 56 subnet mask are possible.

Edit: The question does not mention about continuous/discontinues bit.  If use of continious bits are not allowed then answer will be less then 56.

The question should have been more described. I would have gone for 1 as answer had this been an actual GATE question.

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The question does not mention about continuous/discontinues bit.  If there is restrictions imposed on use of continuous 1 then answer will change.

so, that answer is not wrong. 56 is also not wrong. 1 is also not wrong. It depends on how well described the question is.