Number of subnets question

Hi- I've got the answer to a question but I don't know why the answers are right.Can you help?

The question is ;
The routers in the figure will be configured with an IP address on all interfaces shown in the figure, with RIP-2 being used throughout.The LAN subnets require space for at most 200 hosts per subnet and static length subnet masks should be used throughout.The mask should be chosen to maximize the number of possible subnets.The design engineer intends to use private network 172.30.0.0 for the entire internetwork and use the latrgest numeric values for the subnet numbers. (by the way, the diagram shows that there needs to be 8 subnets)

What I don't understand is the correct answers
The correct answers are

1)if the subnet numbers are chosen well. the design could allow for 248 additional subnets with prefix length /24 (i don't know how they arrive at 248

2) Thew smallest numeric subnet number, based on the requirements, should be 172.30.248.0 /24 (i don't understand how they arrive at 248 )

Can you help me out?

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Comments

tech-airman

Andrew Newman,

What is 200 decimal in binary?

Andrew Newman

200 dec = 11001000 binary

Xenz

248 is used because of the way they want you to design the network. What class is the network you're given? What are they subnetting it to?

How many subnets do you get then in this design? How many hosts can reside in each subnet? Using the last 8 for the network, so the very first would have to be?

This isn't a riddle, answer these and you shall see

EdTheLad

The answers given make no sense,looks like the author hasn't a clue.

Address given is 172.30.0.0 which is class B so starting point is /16 mask.
Maximum 200 hosts per subnet which means no more than 8 bits reserved for the host portion.8 bits will give 254 hosts.
Mask is choosen to maximise the number of subnets, since after allocating 254 hosts per subnet you only have 8 bits left to subnet 2^8=256 subnets.
So a /24 mask is perfect.

billscott92787

Andrew Newman wrote: »

Hi- I've got the answer to a question but I don't know why the answers are right.Can you help?

The question is ;
The routers in the figure will be configured with an IP address on all interfaces shown in the figure, with RIP-2 being used throughout.The LAN subnets require space for at most 200 hosts per subnet and static length subnet masks should be used throughout.The mask should be chosen to maximize the number of possible subnets.The design engineer intends to use private network 172.30.0.0 for the entire internetwork and use the latrgest numeric values for the subnet numbers. (by the way, the diagram shows that there needs to be 8 subnets)

What I don't understand is the correct answers
The correct answers are

1)if the subnet numbers are chosen well. the design could allow for 248 additional subnets with prefix length /24 (i don't know how they arrive at 248

2) Thew smallest numeric subnet number, based on the requirements, should be 172.30.248.0 /24 (i don't understand how they arrive at 248 )

Can you help me out?

I have to say this doesn't make any sense. Since there are 200 hosts required you would definitely have to borrow 8 bits. 2^8 = 256 - 2 = 254 hosts. Which means that you would use a /24 mask. Meaning that you would also borrow 8 bits from the 3rd octet, since this is a class by address, which would give us 256 subnets with 254 hosts per subnet. But, I don't understand how the other answer is correct. It is asking the smallest numeric subnet number based on the requirements, should be 172.30.248.0 /24

The mask is right, but this isn't the smallest subnet. Are there other subnets in the diagram? If so, what are they, it sounds like all the information isn't posted in your question.

mikej412

Andrew Newman wrote: »

The routers in the figure

Um -- okay. Where's the question from (book, chapter/page, question# ) so we can see the figure if we have the book.

Xenz

Andrew Newman wrote: »

Hi- I've got the answer to a question but I don't know why the answers are right.Can you help?

The question is ;
The routers in the figure will be configured with an IP address on all interfaces shown in the figure, with RIP-2 being used throughout.The LAN subnets require space for at most 200 hosts per subnet and static length subnet masks should be used throughout.The mask should be chosen to maximize the number of possible subnets.The design engineer intends to use private network 172.30.0.0 for the entire internetwork and use the latrgest numeric values for the subnet numbers. (by the way, the diagram shows that there needs to be 8 subnets)

What I don't understand is the correct answers
The correct answers are

1)if the subnet numbers are chosen well. the design could allow for 248 additional subnets with prefix length /24 (i don't know how they arrive at 248

2) Thew smallest numeric subnet number, based on the requirements, should be 172.30.248.0 /24 (i don't understand how they arrive at 248 )

Can you help me out?

Highlighted the important parts, but I wanted him to figure it out.

The smallest is relative to the instructions that they want you to use the largest values possible, the last usable subnets. 2^8 = 256 - 8 = 248 being the first (smallest) subnet for use. 256 - 8 reserved for use with a /24 mask leaves 248 for use.

** SPOILER OF SORTS!! **This is probably through the Network Academy as they were always using weird messed up subnets to catch you if you weren't paying attention

Overall, you slept through the subnetting discussion so you should go back and reread it.

And those of you trying to help him should think of this as a good riddle. I just happen to speak nonsense along with English.

billscott92787

I totally agree 100%. I seen an example like this on my Boson practice exams. Is this where you go the question? Like Xenz said they definitely throw these examples at you to catch you off guard. Are you doing this for a networking course or what?

Andrew Newman

Hi, thanks for all the replies.It is off the Boson test that you get with the INCD book.I'll have to get my head around it.
Are you all saying that the question and answer are actually correct?

billscott92787

MAN I'm good. LOL Just kidding, I knew this had to be off there. I know exactly which question you are talking about. That's why I said it had to be off the BOSON exam, because I seen it on the exam. Yes, it is correct. Go back through the Study mode and figure out why. That's the best thing you can do, because then you can fully understand the reason why. At first it is confusing. But, you will get it.

Andrew Newman

Hi Xens et al,
In this design there will be 8 subnets and 256 hosts per subnet.I can now see why 248 should be the smallest subnet number, but I still don't see why it only allows for 248 additional subnets.Is it because they mean 248 APART from the 8 subnets already specified in the network diagram(which I can't copy and paste and you haven't seen)? And do you not do the minus 2 thing because it's RIP2?

billscott92787

Andrew Newman wrote: »

Hi Xens et al,
In this design there will be 8 subnets and 256 hosts per subnet.I can now see why 248 should be the smallest subnet number, but I still don't see why it only allows for 248 additional subnets.Is it because they mean 248 APART from the 8 subnets already specified in the network diagram(which I can't copy and paste and you haven't seen)? And do you not do the minus 2 thing because it's RIP2?

No. You do not do the minus 2 thing for the subnets because you are using "classless addressing" which would be that you assume you are using ip subnet-zero. If this example said "No IP subnet-zero" then you would do the minus 2 for the subnets. The minus two comes into play always with hosts.

Xenz

See my first reply... the IP you're given is a class ____ which is /___ or in dotted decimal ________________. The mask you are subnetting to is /24 or in dotted decimal form _______________. Naturally, this would mean how many subnets can you get out of a /24 mask; ________.

If you cannot provide answers for the blanks, please find the chapter on subnetting.

Andrew Newman

the IP you're given is a class _B___ which is /16___ or in dotted decimal 255.255.0.0________________.
The mask you are subnetting to is /24 or in dotted decimal form 255.255.255.0_______________.
Naturally, this would mean how many subnets can you get out of a /24 mask; _256_(2 to the power 8)______.

Xenz

You could remove the ___.

So 256 subnets - <number you need to use> = ?

You have ? other subnets you could use since it was planned for a /24 mask.

The lowest numbered subnet you can use if you are to use the last 8 subnets available to you with the mask you've chosen = ?

Andrew Newman

I now get that the lowest numbered subnet should be 248 coz 256-8 = 248. I think I was just thinking about it a bit too hard. Thank you for your patience and assistance.