Route summary question

Have a question or want to start a discussion? Post it! No Registration Necessary.  Now with pictures!

Threaded View
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 :(


Re: Route summary question

Quoted text here. Click to load it

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



Re: Route summary question

Cisco Kid wrote:
Quoted text here. Click to load it

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


Re: Route summary question
gregg johnstone wrote:

Quoted text here. Click to load it

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

Quoted text here. Click to load it

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

Quoted text here. Click to load it

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

Quoted text here. Click to load it

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.

Re: Route summary question

Paul Matthews wrote:
Quoted text here. Click to load it

Thanks for that-so the question was slightly incorrect?just hope they
are all error free when I take my CCNA :)


Re: Route summary question
gregg johnstone wrote:
Quoted text here. Click to load it

It's a game. If you are genuinely good enough, the odd point or two from a
dodgy question should not matter<G>

That was what I liked about the old "interview scored" CCIE lab. You could
configure something right, but throw the points away with a silly answer, or
get something nearly there but with the right answer and attitude pull the
points back.

P.
--
Paul Matthews                          
paul@cattytown.me.uk
http://www.hepcats.co.uk

Site Timeline