Subnet question from learntosubnet.com, please help!

Answered Question
Jan 22nd, 2008

So I'm working on the last question under the section "Determine Subnet IDs and Ranges of Host IDs", scenario 10. From another website Mindwerks.com, it's way of teaching you to subnet says to determine the # of bits required to support the total # of subnets, you take the # of subnets needed, and convert it to binary. In scenario 10, they have 3 physical segments, so this in binary is 0000011, or the rightmost 2 bits, the 1 & 2 bit.

Then you take that count of 2 bits, and go from the left to right of the binary chart, and add those bits together, i.e. 128+64=192. This is your subnet mask ID. This has worked for every other question on the learntosubnet.com site, aside from scenario 10. From reading the answer, I can see why they selected the subnet mask, but I don't see how you would determine this without just 'knowing' this is how it works.

Can someone explain this scenario to me a little better? I would expect Cisco would throw something like this example on the CCNA to just confuse me more! I've included the 2 link to the sites I'm reviewing. Thanks in advance, Tony

http://www.learntosubnet.com/Subnetting_Answers.htm

http://www.mindwerks.org/Subneting.htm

Correct Answer by dbeare about 9 years 1 month ago

Scenario 10 says that the required number of physical segments is 5. That is the same as saying that they require 5 subnets. The next line says "Maximum Number of Hosts/Physical Segment" is 25. That tells me that the maximum number of hosts that will be on each subnet is 25 hosts. The smallest subnet size, using these conditions, is 32. That gives me a subnet mask of 255.255.255.224. That subnet mask has 30 usable hosts per subnet, and 6 total subnets (8, if we use ip subnet-zero).

Does that help? They are just using different terminology to describe things. The required number of physical segments is the same as saying that you will need at least X amount of subnets.

HTH!

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Jon Marshall Tue, 01/22/2008 - 14:17

Hi

Okay, i have never done it this way, but if you look at scenario 10 it does not have 3 physical subnets it has 5 physical subnets.

So

00000101 = 3 rightmost bits to represent this

So starting from left

128 + 64 + 32 = 224 which is the answer to solution 10.

Have i misunderstood ?

Jon

dbeare Tue, 01/22/2008 - 14:27

Personally, I think there are probably 100 ways to learn subnetting. Not all of them work for everyone. I tried several different methods before I found the one that worked for me. For the most part, if you have a starting point and know how many networks and hosts there are for that subnet, you can figure out the rest of them fairly simply.

Scenario 10 says the following:

Required # of physical segments: 5

Maximum # of hosts per segment: 25

Network address: 192.177.4.0

Here is how I would work this one out:

I know that the maximum number of hosts per segment that I need to account for is 25. The smallest subnet size that I will need, then, is one that allows for 32 hosts. This would be the /27 subnet, or 255.255.255.224 subnet mask. The easy way to determine the subnet mask for class C subnets is to take 256-(number of hosts), in this case 256-32=224.

The /27 subnet has the following IP ranges:

.0-.31

.32-.63

.64-.95

.96-.127

.128-.159

.160-.191

.192-.223

.224-.255

Now, through theory alone, we are told that you have a maximum of 6 subnets supported, each having 30 hosts (remember, 1 network and 1 broadcast address per subnet), but in the real world you can use the first and last subnets since 'ip subnet-zero' is enabled by default on post 12.0 IOS versions.

HTH.

ttrevino1 Tue, 01/22/2008 - 16:39

The original question only shows the # of segments, the # of hosts/physical segments, and network address, it doesn't give you the subnet mask of .224.

http://www.learntosubnet.com/Subnetting_Problems.htm

So with the question, I don't see where the 5 required subnets is listed on the example, I see 3. Where are you seeing the 5 listed? I may be totally overlooking something in the example.

Correct Answer
dbeare Tue, 01/22/2008 - 18:20

Scenario 10 says that the required number of physical segments is 5. That is the same as saying that they require 5 subnets. The next line says "Maximum Number of Hosts/Physical Segment" is 25. That tells me that the maximum number of hosts that will be on each subnet is 25 hosts. The smallest subnet size, using these conditions, is 32. That gives me a subnet mask of 255.255.255.224. That subnet mask has 30 usable hosts per subnet, and 6 total subnets (8, if we use ip subnet-zero).

Does that help? They are just using different terminology to describe things. The required number of physical segments is the same as saying that you will need at least X amount of subnets.

HTH!

ttrevino1 Wed, 01/23/2008 - 05:51

The way you described it with 5 required subnets does make sense, and I can figure it correctly the way these 2 sites have said to. But either I don't understand subnetting as well as you do yet, or I must be seeing something different on my copy of the question and answer pages, as I see printed on the site "Number of Physical Segments: 3". I'm assuming you're seeing it printed as 5? That would make better sense, so maybe there's just something going on with my browser or something that is showing a different number of segments.

Anyway, thanks for explaining it, it does make sense now. I appreciate the help.

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