What to do first?

osJoe · December 2011

When dealing with the types of questions that show you a network topology and ask multiple questions (most likely dealing with subnetting) what is the best step to take first? I have a habit of going straight to the possible answers and going through each one of them?

Here is an example..

Refer to the exhibit. The goal of this network design is to provide the most efficient use of IP address space in a network expansion. Each circle defines a network segment and the number of users required on that segment. An IP subnetwork number and default gateway address are shown for each segment.

What are three problems with the network design as shown? (Choose three)

A – Interface fa0/3 has an IP address that overlaps with network 10.1.3.0/30.
B – Interface fa0/1 has an invalid IP address for the subnet on which it resides.
C – Interface fa0/2 has an invalid IP address for the subnet on which it resides.
D – Network 10.1.2.0/25 requires more user address space.
E – Network 10.1.3.128/25 requires more user address space.
F – The IP subnet 10.1.1.0/30 is invalid for a segment with a single server.

instant000 · December 2011

osJoe wrote: »

When dealing with the types of questions that show you a network topology and ask multiple questions (most likely dealing with subnetting) what is the best step to take first? I have a habit of going straight to the possible answers and going through each one of them? Here is an example.. Refer to the exhibit. The goal of this network design is to provide the most efficient use of IP address space in a network expansion. Each circle defines a network segment and the number of users required on that segment. An IP subnetwork number and default gateway address are shown for each segment. What are three problems with the network design as shown? (Choose three)

A – Interface fa0/3 has an IP address that overlaps with network 10.1.3.0/30. B – Interface fa0/1 has an invalid IP address for the subnet on which it resides. C – Interface fa0/2 has an invalid IP address for the subnet on which it resides. D – Network 10.1.2.0/25 requires more user address space. E – Network 10.1.3.128/25 requires more user address space. F – The IP subnet 10.1.1.0/30 is invalid for a segment with a single server.

I looked at the pictures, and saw problems with three of the networks proposed. I then started to compare them to the answers given.

However, if I was delayed in seeing an answer, I would just look at the answer choices, and compare them to the picture. That would be a quick way to do it, as you don't have to worry about translating what you saw into an answer (and possibly missing one).

I seem to have lost my formatting. Maybe it is browser-based, and the lack of script in this one is why I don't have full editing here. ... will need to do a test. (EDIT: Yep, browser-based. The formatting function appears to rely on javascript... makes sense.)

Hope this helps!

these are my answers, and why:

FA0/4 - 130 users, the mask covers it, and the IP of the interface is within that network, no issues
FA0/1 - The interface address is not valid for the network given. The mask is ok. Only 2 addresses needed, see no problem with the mask.
FA0/2 - The interface address is valid for the network. however, the mask given won't support 130 users
FA0/3 - The interface address does not match the network. The mask is ok. The network will support the number of users.

So, comparing that to the exhibit choices.
A -
B -
D -

A was actually a poorly-phrased choice, but it is about the best one I can come up with here. I don't like it, but it feels to be the best answer, to match the problems that I saw with the exhibit.

osJoe · December 2011

Thanks so much for working it out! I understand them all except A. I agree, very confusing. I guess since it "belongs" to that subnet that it would be overlapping and be invalid.

Todd Burrell · December 2011

Generally I tend to attack problems like this by looking at the diagram and then I try to see where the mistakes are located. So obviously the top network does not work because the IP address used for the interface is not in the network listed. You can attack the other portions of the network shown the same way to insure that the subnet masks completely cover the number of users listed. Here are the issues you will usually find from a diagram:

1) The network interface is not in the subnet listed.
2) The network interface is using the network address as its IP address.
3) The network interface is using the broadcast address as its IP address.
4) The network/mask does not give enough IP addresses for the users indicated.

I normally try to quickly get the address ranges for the subnets and then you can quickly see problems with interface addresses.

Hope this helps.

osJoe · December 2011

Thank you Todd, I will definitely go through those steps when these questions come up!

Turgon · December 2011

osJoe wrote: »

When dealing with the types of questions that show you a network topology and ask multiple questions (most likely dealing with subnetting) what is the best step to take first? I have a habit of going straight to the possible answers and going through each one of them?

Here is an example..

Refer to the exhibit. The goal of this network design is to provide the most efficient use of IP address space in a network expansion. Each circle defines a network segment and the number of users required on that segment. An IP subnetwork number and default gateway address are shown for each segment.

What are three problems with the network design as shown? (Choose three)

A – Interface fa0/3 has an IP address that overlaps with network 10.1.3.0/30.
B – Interface fa0/1 has an invalid IP address for the subnet on which it resides.
C – Interface fa0/2 has an invalid IP address for the subnet on which it resides.
D – Network 10.1.2.0/25 requires more user address space.
E – Network 10.1.3.128/25 requires more user address space.
F – The IP subnet 10.1.1.0/30 is invalid for a segment with a single server.

TCP/IP has a logic to it. Understand that well first then look at the scenario as described. From there the invalid answers will be obvious.

What to do first?

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