﻿Cisco: subnet calculation - cisco.questionfor.info

subnet calculation

3,448 words with 2 Comments; publish: Sun, 01 Jun 2008 09:49:00 GMT; (400171.88, « »)

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 - 255.255.0.0

B - 255.255.240.0

C - 255.255.255.0

D - 255.255.255.192

E - 255.255.255.224

I know the answer is D (255.255.255.192), but i would really like to

know how i could find out why D is the correct answer.

Thanks for any help.

• w00 wrote:

> 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 - 255.255.0.0

> B - 255.255.240.0

> C - 255.255.255.0

> D - 255.255.255.192

> E - 255.255.255.224

> I know the answer is D (255.255.255.192), but i would really like to

> know how i could find out why D is the correct answer.

> Thanks for any help.

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. 255.255.255.192 leaves you with 62 useable hosts which is more

than you need.

Cheers!

D~

http://www.telecom-gear.com

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#1; Sun, 01 Jun 2008 09:50:00 GMT
• "Drake" <drake.cisco.questionfor.info.nola.net> wrote in message

news:45c4d227\$0\$16355\$88260bb3.cisco.questionfor.info.free.teranews.com.. .

> "w00" <AHIsDumb.cisco.questionfor.info.gmail.com> wrote in message

> 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)

>

FYI...

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.

#2; Sun, 01 Jun 2008 09:51:00 GMT