Route summary question

Route summary question
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Subject Author Date
Route summary question gregg johnstone 09-07-2006
Posted by gregg johnstone on September 7, 2006, 4:09 pm
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Hi
I know this was covereda while back , and I thought I knew how to do
it, but I just tried a Semester 3 quiz and got this quetion wrong

Summarize these two routes

172.21.136.0 /24
172.21.143.0 /24

The answer wa 172.21.136.0 /20

I worked it out as 172.21.136.0 /21

because the third octet worked out as
|
10001|000 -136
10001|111 -143
|
so i used the fifth - /21

Please help,as I thought I had this figured out :(


Posted by Cisco Kid on September 8, 2006, 1:04 am
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> Hi
> I know this was covereda while back , and I thought I knew how to do
> it, but I just tried a Semester 3 quiz and got this quetion wrong
>
> Summarize these two routes
>
> 172.21.136.0 /24
> 172.21.143.0 /24
>
> The answer wa 172.21.136.0 /20
>
> I worked it out as 172.21.136.0 /21
>
> because the third octet worked out as
> |
> 10001|000 -136
> 10001|111 -143
> |
> so i used the fifth - /21
>
> Please help,as I thought I had this figured out :(

You are correct. The 1st five bits of the 3rd octet can be summarized so
16+5=21.



Posted by gregg johnstone on September 8, 2006, 3:26 am
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Cisco Kid wrote:
> > Hi
> > I know this was covereda while back , and I thought I knew how to do
> > it, but I just tried a Semester 3 quiz and got this quetion wrong
> >
> > Summarize these two routes
> >
> > 172.21.136.0 /24
> > 172.21.143.0 /24
> >
> > The answer wa 172.21.136.0 /20
> >
> > I worked it out as 172.21.136.0 /21
> >
> > because the third octet worked out as
> > |
> > 10001|000 -136
> > 10001|111 -143
> > |
> > so i used the fifth - /21
> >
> > Please help,as I thought I had this figured out :(
>
> You are correct. The 1st five bits of the 3rd octet can be summarized so
> 16+5=21.

thanks for that -but the answer on the cisco quiz was /20 ? was it just
an error.?


Posted by Paul Matthews on September 8, 2006, 3:29 am
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gregg johnstone wrote:

>172.21.136.0 /24
>172.21.143.0 /24

Both /24s so no nasty bits in the last octet. all we need worry about is the
third as that is all that differs.

>I worked it out as 172.21.136.0 /21
>
>because the third octet worked out as
> |
>10001|000 -136
>10001|111 -143
> |

Your binary conversion is correct, and your observation is also correct. Five
bits is the commonality.

>so i used the fifth - /21
>
>Please help,as I thought I had this figured out :(

You have. You have listened well on here. Now for the "correct" answer.

>The answer was 172.21.136.0 /20

That is a mismatch between route and mask. 172.21.136.0 is a host address if
the mask is /20.

1000|1000 -136

The correct network ID from that mask and address is 172.21.128.0.

This is where I dislike computer based testing - if the question is wrong, you
may be correct but you lose the mark. At least with the old proctor scored CCIE
lab test you could argue your point with a human, in fact I think there were
some parts where they actually wanted you to...

P.
--
Paul Matthews CCIE #4063
Please post questions to the NG, NOT by e-mail.

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