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Number of subnets question
Andrew Newman
Member Posts: 23 ■□□□□□□□□□
in CCNA & CCENT
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?
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?
Comments
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OptionsXenz Member Posts: 140248 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 seeCurrently working on:
CCNP, 70-620 Vista 70-290 Server 2003
Packet Tracer activities and ramblings on my blog:
http://www.sbntech.info -
OptionsEdTheLad Member Posts: 2,111 ■■■■□□□□□□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.Networking, sometimes i love it, mostly i hate it.Its all about the $$$$ -
Optionsbillscott92787 Member Posts: 933Andrew 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. -
Optionsmikej412 Member Posts: 10,086 ■■■■■■■■■■Andrew Newman wrote: »The routers in the figure:mike: Cisco Certifications -- Collect the Entire Set!
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OptionsXenz Member Posts: 140Andrew 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.Currently working on:
CCNP, 70-620 Vista 70-290 Server 2003
Packet Tracer activities and ramblings on my blog:
http://www.sbntech.info -
Optionsbillscott92787 Member Posts: 933I 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?
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OptionsAndrew Newman Member Posts: 23 ■□□□□□□□□□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? -
Optionsbillscott92787 Member Posts: 933MAN 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.
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OptionsAndrew Newman Member Posts: 23 ■□□□□□□□□□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? -
Optionsbillscott92787 Member Posts: 933Andrew 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. -
OptionsXenz Member Posts: 140See 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.Currently working on:
CCNP, 70-620 Vista 70-290 Server 2003
Packet Tracer activities and ramblings on my blog:
http://www.sbntech.info -
OptionsAndrew Newman Member Posts: 23 ■□□□□□□□□□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)______. -
OptionsXenz Member Posts: 140You 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 = ?Currently working on:
CCNP, 70-620 Vista 70-290 Server 2003
Packet Tracer activities and ramblings on my blog:
http://www.sbntech.info -
OptionsAndrew Newman Member Posts: 23 ■□□□□□□□□□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.
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Optionsphoeneous Member Posts: 2,333 ■■■■■■■□□□Andrew Newman wrote: »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.
Dont feel bad, the question is confusing.