MS Press Subnetting Question

I’m reading through the MS Press book for this test and got to page 2-54 and just don't understand what is going on with the question. Can one of you who "see the matrix" give me a better explanation as to why this scenario is improperly configured?

Thanks!

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Claymoore

With a /27 subnet mask (255.255.255.224 for those of you who don't speak CIDR), both of the 'networks' on either side of the router are actually on the same subnet.

Some people do this math a little different, but you need to convert everyting to binary. This method works for me and I consider a subnet to have a value of 256 (0-255 is 256 values because 0 is a number) and that makes it easy to get a subnet mask from a /xx CIDR value.

Mask:
32-27=5
2^5=32
256-32=224 (subnet mask)

Network Range for 4th octet
32-27=5
2^5=32
32 addressess per subnet (30 usable hosts)
0-31
32-63
64-95
96-127
128-159
160-191
192-223
....

The correct subnet mask is /28 or 255.255.255.240
Mask:
32-28=4
2^4=16
256-16=240

Range (4th Octet)
32-28=4
2^4=16
16 addresses per subnet (14 usable hosts)
0-15
16-31
32-47
48-63
64-79
80-95
96-111
112-127
128-143
144-159
160-175
176-191
....

Now that the subnet mask is correct, the networks on either side of the router will really be on different subnets. Why does this matter? If the subnet mask is not correct then Host A thinks Host C is on the same network and just sends the packet out to the local subnet rather than the default gateway. Now that the subnet mask is correct, Host A will send a packet destined for Host C to the default gateway and let the router route it.

Sanger80

OMG, thank you so much for the reply. The lights just went on upsairs! I was totally stuck sitting there staring at that question!!!

Claymoore

You're welcome, glad I could help.

Actually, in the Cisco world at least, you wouldn't have to worry about making this mistake. The router would reject the configuration because the interfaces overlap in the same network (subnet). I don't know if Server 2003 would allow this as a routing configuration either - I haven't gone that far in my 291 studies yet.

Sanger80

That's a good question. When I get to the RRAS part of the book I will have to give that a try. Thanks again. You have no idea how long that question was bugging me. I wish MS Press would offer up explanations that are a bit more elaborate. That was the only one that gave me a problem on that chapter, and for some reason I was not looking at it the right way. I think my brain is getting information logged.......

Sanger80

Ok, I just worked out the next question on page 2-54.

/21 Address (255.255.248.0)

32-21=11
2^11=2048 adresses per subnet (2046 usable)

2^5=32 Subnets

2^3=8 Subnet Address Range

0-7
8-15
16-23
.
.
.
56-63 So right here I know Client A and Client B are on the same subnet and can talk.
.
.
.
120-127 Ok, here is the red flag. Client C is using this subnet and Client D is in the next subnet (128-135), so they cant talk.

One thing that I also didnt have worked out in my head was how to determine a subnet address range. Now I know that you take the bits used in the octet you are subnetting that are not used for the network, in this case 3 bits and then calculate r=2^b where r is the address range and b represents the bits left for hosts in the octet you are subnetting. In this case 3 bits, so 2^3=8 so I know that the subnets are going to be in ranges of 8. Nice. I am starting to be able to do this in my head. Its about time.

Thanks again Claymoore. You made my weekend.

Claymoore

Yes, it is that easy. A few years ago I discovered that subnetting can be learned in 10 minutes. Unfortunately it took me two months of studying TCP\IP for the old NT 4.0 exams to discover that....

shednik

Claymoore wrote:

Yes, it is that easy. A few years ago I discovered that subnetting can be learned in 10 minutes. Unfortunately it took me two months of studying TCP\IP for the old NT 4.0 exams to discover that....

Yea i'm just beginning my MS exams and i'm definatley glad i have my ccna since subnetting will be a complete breeze for me.