Subnetting Question

CiscoCerts

I'm working some problems off of learntosubnet.com and I have a problem with the wording and/or answers. Here's one:

Scenario 5
Required Number of Physical Segments: 16
Maximum Number of Hosts/Physical Segment: 1,500
Network Address: 128.199.0.0

Their answer:
Proposed Custom Subnet Mask: 255.255.248.0
Number of Subnets Supported: 30
Maximum Number of Host IDs per Subnet: 2,046

My answer
Proposed Custom Subnet Mask: 255.255.252.0
Number of Subnets Supported: 64 (they will say 62)
Maximum Number of Host ID per Subnet: 1022

I know that they're using the old method of still subtracting two from the network addresses, which you do not have to do anymore on the current tests. So that's why i put they will say 62.

Maybe I'm reading this wrong but the question says Maximum Number of Hosts per subnet is 1500, and they answer given more than 1500? when its the max? It doesn't say its the least amount required.. and it does say its the least amount required for the physical.

So who's right here? Help me understand this if I'm wrong

Thanks guys.

CCENT Scheduled for next Friday

JayrodEF

Well, first of all, your answer does not meet the max number of hosts/subnet requirement. Their answer is stating 2,046 max host per subnet which is what you'd get if you added one more bit to the host address part of the address. your only options are 1022 host/subnet if you have 10bits for the host address. Then it goes up to 2046 hosts/subnet if you have 11bits for the host address. The closest one that allows up to (and beyond) 1500 hosts/subnet is if you use 11 bits for the host address portion. That's why their anwer is correct. You must alway encompass the max number of hosts in the question, even if the correct answer allows for even more. So, if you have 11 bits for host address, that leaves you with 21 bits for the network/subnet address portion and gives you a subnet mask of 255.255.248.0. That's the only option that satisfies both the max hosts per subnet and number of subnet requirements. Hope that helps.

dynamik

Yea, even though they may not use more than 1500, you still need to choose the option that provides at least that amount.

You should be given clues in the question whether or not you have to subtract two from the networks. If they didn't provide any, don't worry about it, it was just a poorly written question.

CiscoCerts

JayrodEF wrote:

Well, first of all, your answer does not meet the max number of hosts/subnet requirement. Their answer is stating 2,046 max host per subnet which is what you'd get if you added one more bit to the host address part of the address. your only options are 1022 host/subnet if you have 10bits for the host address. Then it goes up to 2046 hosts/subnet if you have 11bits for the host address. The closest one that allows up to (and beyond) 1500 hosts/subnet is if you use 11 bits for the host address portion. That's why their anwer is correct. You must alway encompass the max number of hosts in the question, even if the correct answer allows for even more. So, if you have 11 bits for host address, that leaves you with 21 bits for the network/subnet address portion and gives you a subnet mask of 255.255.248.0. That's the only option that satisfies both the max hosts per subnet and number of subnet requirements. Hope that helps.

I take maximum to mean "not to exceed"

Do not exceed 1500 hosts. Their answer does. Isn't this confusing?

JayrodEF

Yeah, those are worded weird. Usually that means that segment must be able to handle at least 1500 hosts. The only way to handle 1500 hosts is to use a subnet mask that actually allows 2046 hosts.

CiscoCerts

You are correct. And it should say least. This makes their answer wrong. The question sucks and i'm not doing any more from there!!!


Thanks for the confirmation it was driving me crazy.

tech-airman

CiscoCerts,

CiscoCerts wrote:

I'm working some problems off of learntosubnet.com and I have a problem with the wording and/or answers. Here's one:

Scenario 5
Required Number of Physical Segments: 16

The key word is "Required." That means you MUST have AT LEAST 16 "Physical Segments" which translates to subnets.

CiscoCerts wrote:

Maximum Number of Hosts/Physical Segment: 1,500

Since the key word "Required" is not present, concentrate on a subnet mask that will satisfy the above requirement of 16 subnets. If this is a well written question, then by satisfying the 16 subnet requirement, you should also satisfy this requirement.

CiscoCerts wrote:

Network Address: 128.199.0.0

Any subnetting problem would be difficult to solve without being given the network address.

CiscoCerts wrote:

Their answer:
Proposed Custom Subnet Mask: 255.255.248.0
Number of Subnets Supported: 30
Maximum Number of Host IDs per Subnet: 2,046

My answer
Proposed Custom Subnet Mask: 255.255.252.0
Number of Subnets Supported: 64 (they will say 62)
Maximum Number of Host ID per Subnet: 1022

I know that they're using the old method of still subtracting two from the network addresses, which you do not have to do anymore on the current tests. So that's why i put they will say 62.

Maybe I'm reading this wrong but the question says Maximum Number of Hosts per subnet is 1500, and they answer given more than 1500? when its the max? It doesn't say its the least amount required.. and it does say its the least amount required for the physical.

So who's right here? Help me understand this if I'm wrong

Thanks guys.

CCENT Scheduled for next Friday

From a designing perspective, when they mention stuff like "Maximum number of Host ID per Subnet," that's the maximum number of PLANNED host IDs not the ACTUAL number of host IDs. When some people and/or organizations decide to "think outside the box" and try to exceed the number of planned host IDs, some hosts may have connectivity issues because there's IP address exhaustion for that subnet. When that happens, you should resubnet the entire network and implement that IP addressing scheme into the entire network instead of some sort of "band-aid" solution to the problem.

I hope this helps.

snadam

to further strengthen your point, I took "required number of subnets: 16" literally (meaning no more and no less), and worked the problem as such. Which would mean the subnet mask I would be using is 255.255.240.0. which would give me 16 subnets on the mark, and 4094 hosts per subnet.

I agree that its poorly worded question. Also, I noticed that they are still subtracting 2 from the subnet side : ; which hasn't been done in ages...

aragoen_celtdra

You know what? I had the same exact qualms about the wording of this problem. But get used to it. I've done a lot of subnetting practices since I started studying for CCENT (5-10 practice problems almost everyday since Jan ). Almost every resources I used have then worded this way. But you get used to the wording. In fact it makes lot of sense to me now and evertime I see a question about subnetting worded that way (i.e "you need a maximum of x subnets and y hosts) I go on automatic mode and know exactly what it's asking and how to solve it.

Sometimes you just need to do a lot of them to make them stick and for them to eventually make sense. So keep practicing, analyze, and ask questions if you're not sure

1MeanAdmin

Maybe it's just me but I see no problem when looking at the question.

Required Number of Physical Segments: 16

Their answer allowed 30: more than required.
Imagine this: If 850 is required to pass CCNA, does it mean you will fail with 900?
Required = Minimum.

Maximum Number of Hosts/Physical Segment: 1,500

That means "Maximum number of hosts you will ever have on a subnet". 2,046 or 4,094 or 8,190 will work but the maximum you will ever need is 1,500. So 2,046 is the best option.

EDIT: Good rule of thumb is to start by allowing minimum number of hosts per subnet to accomplish the task (2,046) and dedicate the rest of the bits for subnets (which is usually more than enough).

Hope this helps.