Subnetting Problem

I need some help with the following subnet problem:

"You are the new administrator of a 2000 node network.
There is only one router on the entire network, which
provides all the computers with Internet access. The
company's ISP has assigned the following 8 network
addresses to them:

10.24.32.0/24
10.24.33.0/24
10.24.34.0/24
10.30.35.0/24
10.30.36.0/24
10.30.37.0/24
10.30.38.0/24
10.30.39.9/24

What subnet mask could you use to minimize the complexity
of the routing tables while maintaining the existing
Internet connectivity?

a. 255.255.252.0
b. 255.255.255.252
c. 255.255.255.248
d. 255.255.248.0 "

The answer is 'd.' Quite frankly I do not understand what
the problem is getting at nor how 'd' gives the desired
results.

Any explainations would be appreciated.

 

"/24" will tell you that the SNM is 255.255.254.0, but
not 255.255.248.0 (this is "/21").

 

D. 255.255.248.0
BTW, /24 subnet mask is 255.255.255.0

 

I thought the same thing. That /24 indicates a
255.255.255.? mask. So I figured that either b or c was
the answer. But according to the MS 70-216 trng kit pg 808
it is d.

I understand the basics of subnetting and figured out
correctly the other questions in this section but I do not
understand the logic of this question.

 

You "borrow" from the Host address to increase the client address range. You
really need to take the advice to visit the referenced site
(learntosubnet.com), lots of good stuff there!

 

Either the question has been mis-typed here or mis-remembered.
255.255.248 won't help cover all eight of those addresses, which jump
from 10.24.*.* to 10.30.*.*. You'd need at least a /13 (255.248.0.0)
mask to cover all those.

The question is also bogus because 10.*.*.* is private NAT space, and
would not be allocated by an ISP -- but even overlooking that, the
suggestion that you could simply apply one large mask to the whole
network and call it a day is specious at best. If you were to do so,
then people on this network would be unable to reach people using the
same ISP from elsewhere, i.e. 10.27.5.0/24 would be unreachable.

BAD question, nice troll.

 

Unless my calculations are incorrect, subnet mask 255.255.248.0 will allow
2048 or 2046 host addresses using a class A address to start.

 

Jeffrey Woods has provided the best answer so far.

Your ISP may not be a total idiot for "assigning" this IP
space in the first place. It would only work on your
10.24or30.y.z network and possibly to reach the ISP's
servers / routers and that is likely it. However, if the
ISP provides mail and proxy servers (possibly other
services?) then this IP assignment is OK.

10.24.x.y to 10.30.a.b would require a mask of 255.248.0.0
to cover all those IP ranges mentioned.

The 10.24.x.y ranges could be covered by "route add
10.24.32.0 mask 255.255.253.0 (gateway IP here)"

....Assuming 10.30.39.9/24 is a typo and should be
10.30.39.0/24 (cause the first is definitely wrong and
does not exist)...

Then route add 10.30.35.0 mask 255.255.251.0 (gateway IP)

So far, neither route table addition looks like our answer.


HOWEVER, if you concede that it is highly probable that
all the class C subnets (those finishing with /24) start
with either 10.24 OR 10.30, then 10.x.32.0 with a mask of
255.255.248.0 will cover all IPs in the range 10.x.32.0 to
10.x.39.255. Making "d" the correct choice....


It it says d is right, then I would suggest the above
paragraph is the solution.

Anyone else care to have a go?

....care to kudos?

....care to flame?

Marko Cosic

 

I have something to say here.

Let me try an answer for this

This questions involves something called "ROUTE Summarization".

10.24.32.0/24
10.24.33.0/24
10.24.34.0/24
10.30.35.0/24
10.30.36.0/24
10.30.37.0/24
10.30.38.0/24
10.30.39.9/24

these are the IP Addresses given. The questions says "What subnet mask could you use to minimize the complexity of the routing tables while maintaining the existing Internet connectivity?"

so instead of just sub-netting you have to take care of minimizing the routing table it generates.

So we have to make this pool of IPs into two groups. 10.24 and 10.30 group.

10.24 has 3 IPs in the pool
10.30 has 5 ips in the pool so we have to consider grouping 5 IP pool to minimize the routing table.

10.30. 35-39 is the range - group of 8 ip address will accommodate this range
let me do this in binary

To get a 8 ip range we have to start from the 5th binary digit 11111000=248 but we need this 248 in the 3rd octet mask to accomplish the range we needed because first two octets are same ( 10.30.)

so the mask should be 255.255.248.0.

eventually,
10.30.35.0/24
10.30.36.0/24
10.30.37.0/24 ---->255.255.248.0 is the mask you need
10.30.38.0/24
10.30.39.9/24

For 10.24 series you can use the same mask because it lies within the range but route summarization is not needed as there are only 3 IPs in this range.

What do you guys think?

 

I really recommend using this too to do route summarize: http://www.subnet-calculator.org/supernets.php