VLSM

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192.168.1.64/26 192.168.1.128/26

The two routers have been configured with the "no ip subnet zero" command. Which of the following VLSM network addresses can be used between the routers? (Choose two answers.)

a) 192.168.1.0/30 b) 192.168.1.4/30 c) 192.168.1.8/30 d) 192.168.1.96/30 e) 192.168.1.138/30 f) 192.168.1.190/30

Correct answers are B and C.

It is my understanding that when working with VLSM, one starts by using smaller masks and working up (more hosts to less hosts). Since the LANs are using /26 (.0 -> .63, .64 -> .127, .128 -> .191. and .192 -> .255) and the "no ip subnet zero" command is being used on both routers, subnet #0 (.0 -> .63) is not available. How can answers B and C be correct?

I wrote Cisco and received the following answer:

"The issue would be valid if a 255.255.255.192 (/26) mask was used with either the 192.168.1.4 or 192.168.1.8 addresses, however with VLSM, the /30 mask creates a new series of subnets within the 192.168.1.0 through 192.168.1.63 range, all of which can be used with the exception of the 192.168.1.0/30 subnet unless the ip subnet-zero use is enabled."

Can anyone clarify this for me?

Many thanks!

Joey Lanza

Reply to
J. Lanza
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Hi Joey,

Their answer is correct, the .0 sunbet is out because of the "no ip sunbet zero" otherwise that would be correct as well. Thing to keep in mind is that most all tests want you to conserve addresses which would mean using the highest bit mask possible. There is nothing that requires you use the same mask for the entire block when using VLSM, as long as you don't cross the subnet boundry. You can carve and choose however you see fit.

For instance you could have in the 192.168.1.X range, all of which are valid:

192.168.0.0/29 which would be hosts 1-6 192.168.0.8/30 which would be hosts 9-10 192.168.0.12/30 which would be hosts 13-14 192.168.1.16/28 which would be hosts 17-30 192.168.1.32/28 which would be hosts 33-46 192.168.1.48/28 which would be hosts 49-62 192.168.1.64/26 which would be hosts 65-126 192.168.1.128/25 which would be hosts 129-254

-Brian

Reply to
Brian V

I understand the source of your confusion. What you are missing is that whether a particular subnet is an all-zeros subnet or not also depends on the mask. So, for instance given just the IP address

192.168.1.5, you can't tell if it is part of an all-zero subnet or not. You would need additional information about the mask to make that determination. Now, if you were told that it was 192.168.1.5/26, it would be part of an all-zeros subnet (since the subnet is 192.168.1.0/26). However, if you were told that it was 192.168.1.5/30, it would not be part of an all-zeros subnet (since the subnet is 192.168.1.4/30).

In the particular example you quote, you are continuing to think in terms of the /26 when you should be thinking in terms of the /30 prefix. Remember that the basic definition of an all-zeros subnet is that it should have all zeros for the subnet bits when the address is written in binary. To determine the subnet bits, you need to use the right mask and that is why the mask is important in determining whether a subnet is an all-zeros subnet or not.

Hope this clarifies a bit.

Cisco da Gama

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Reply to
ciscodagama

Hi Joey ,

The correct answer is B and C. b) 192.168.1.4/30 c) 192.168.1.8/30

Explanation.

subnetting 192.168.1.64/26 we get the following networks.

192.168.1.4 192.168.1.8 192.168.1.12 192.168.1.16 192.168.1.20 192.168.1.24 192.168.1.28 192.168.1.32 192.168.1.36 192.168.1.40 192.168.1.44 192.168.1.48 192.168.1.52 192.168.1.56 192.168.1.60 192.168.1.64

since no ip subnet zero is entered, you cannot use zero subnet avaliable usable subnet is ..192.168.1.4 and 192.168.1.8

thanks. dabance CCNP

J. Lanza wrote:

Reply to
dabance

i'm glad you got some clarifications there. I wonder if anybody, can answer this one, a more fundamental simple question regarding that orignal qusetion. 'cos i don't get it!!

where is the q from?

IPs are on interfaces, right, surely we agre there? ;) even the IP given to a router to access it remtely.

the 2 interfaces facing each other, connected up, should be on the same subnet, shouldn't they?

but 192.168.1.64/26 192.168.1.128/26 VLSM, are different subnets.

I get closer to the qs answer. If the Q were.

1 router interface is 192.168.1.64/26 VLSM. Another is 192.168.1.128/26 . Thoe interface are not connected. there are other routers and other interfaces on the NW. and the q was what other subnets can be used

but even then.

a) is subnet zero. so not allowed. So not a good ans b) 192.168.1.4/30

Reply to
q_q_anonymous

The 192.168.1.64/26 and the 192.168.1.128/26 subnets are on other interfaces on the two routers. The 64/26 subnet is using addresses 64-127 The 128/26 subnet is using addresses 128-191.

Answer D uses address 96 which is being used in subnet 64/26 Answer E uses address 138 which is being used in subnet 128/26 Answer F uses address 190 which is being used in subnet 128/26

Reply to
Alan S

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