subnet calculation

Hey, i'm making a test for cisco (CCNA01), but im at a question where i have to calculate subnets. But im not sure how to do this.

The question is as follows: A company with a Class B license needs to have a minimum of 1000 subnets with each subnet capable of accommodating 50 hosts. Which mask below is the appropriate one?

A - B - C - D - E -

I know the answer is D (, but i would really like to know how i could find out why D is the correct answer. Could someone please help me out with the calculations.

Thanks for any help.

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because you need at least all the third octet (255) times 4 (the borrowed two bits of the fourth one) to accommodate for at least the 1000 subnets you need. leaves you with 62 useable hosts which is more than you need.

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While 2^n-2 still applies for number of hosts, that is incorrect to use that formula for the number of subnets. With ip subnet-zero enabled (the default), you can use the lower and upper subnets, which gives you 2^n. You may have pulled that information from some older Cisco documentation, as that used to be the correct answer. Go figure.

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You have a class B address or /16 address so you have

16 bits available for subnetting. a simple formula to help you determine how many bits to use for subnets and hosts is: 2^n -2

This implies you need 10 bits for 1000 subnets (2^10 - 2 = 1022) and the remaining 6 bits will give you enough hosts (2^6 -2 =62) to cover the stated requirement.

Your subnet mask will be /26 or (11111111 11111111 11111111 11000000) 255 . 255 . 255 . (128+64)

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