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What subnet mask to use
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| OK. Question time.
I got this question wrong when I tried it.
Q. Given a class B address, what subnet mask do I need to split the class B into 8 class A addresses.
More importantly, why?
Go on, have a go!
Hippo
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| /13 or 255.248.0.0
Assuming subnet zero is enabled.
You've borrowed three bits from the network address, because 2n is 2*2*2=8
As you want a class A you take the mask backwards???
Is that what you should have, and more importantly do you understand???
If I remember right there are a couple of questions like this in the routing exam cert guide and they got me first time!!
PM me if I'm wrong, with what you think the right answer is and I'll have another go if your trying to get people thinking and don't want to post the answer!!!! | |
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|  Mat
I am sure your correct with answer, however, I was so confused when I saw the question on the cert guide, that I even reproduced it here INCORRECTLY  
Here's what I should have written:
OK. Question time.
I got this question wrong when I tried it.
Q. Given a class B address, what subnet mask do I need to split the class B into 8 CLASS C addresses.
More importantly, why?
Yeah! I really meant 8 CLASS C addresses; and I am still trying to figure it out. Some more help needed please, to amke the light in my head go on.
Cheers
Hippo | |
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| lol - it's getting late.
ok, try again.....
I would have thought it would just be 255.255.224.0
Again assuming subnet zero is allowed...
Same reason as before, using three bits as 2*2*2=8, giving subnets of :
0, 32, 64, 96, 128, 160, 192, and 224.
(All calculated using 256-224=32), and simply starting at zero and adding 32 on repeatedly.
If your still not sure have a look at a few of the subnetting guides around, freaks seems to be highly recommended recently. | |
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| Mat
Exactly what I calculated it to be, 255.255.224.0; however the cert guide gives the answer as 255.255.248.0.
I took a dummy address of 140.100.0.0 and put it into my subnet calculator, used255.255.248.0 and out popped 32 subnets, or CLASS C addresses.
I trust your answer, but it would reassuring to hear from anyone else on this topic.
Cheers
Hippo | |
| dmaftei 2002-02-15, 4:58 pm |
| quote:
Originally posted by Hippo
Exactly what I calculated it to be, 255.255.224.0; however the cert guide gives the answer as 255.255.248.0.
That's because whoever put that answer in the cert guide is stupid (or sloppy, or careless, or brainless... pick what you like). You and Mat are right. | |
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| Nice one Dan & Mat!
Got this MSFB nailed down now. I will no longer trust the answers the cert guide gives; which means I have to know my stuff inside out, and mail the buggers when THEY are wrong.
Cheers
Hippo | |
| The Reamer 2002-02-15, 8:38 pm |
| Have you checked for the errata for the book? I know some folks swear by CiscoPress, but they make mistakes as well.
Reamer | |
| dmaftei 2002-02-15, 8:52 pm |
| quote:
Originally posted by Hippo
Got this MSFB nailed down now.
What the heck is a MSFB?! | |
| Hippo 2002-02-16, 12:52 am |
|  I'll leave you to fill in the 'F' & 'B'
Mother F Son of a B
Hippo (Time to go)
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| Yankee 2002-02-17, 7:03 am |
| quote:
Originally posted by dmaftei
That's because whoever put that answer in the cert guide is stupid (or sloppy, or careless, or brainless... pick what you like). You and Mat are right.
How many host addresses are available in 8 class C blocks and how many are in a 255.255.248.0 block?
Yankee | |
| dmaftei 2002-02-17, 7:30 am |
| quote:
Originally posted by Yankee
How many host addresses are available in 8 class C blocks and how many are in a 255.255.248.0 block?
Yankee
2032 and 2046 respectively. Why do you ask (since I'm sure you know the answer already)?! And what's the connection between this question and the quote?! | |
| strikeattack 2002-02-17, 7:12 pm |
| quote:
What the heck is a MSFB?!
The MSFB is the successor to the 6000 and 6500s MSFC card. It provides a larger on-board cache, faster CPU, more p/s, and higher scalability. | |
| RTRGOD 2002-02-20, 10:55 am |
| Guys, the terminology on the question seems a little off to me? Did you give us the question as it reads from the book? To me, a class C address is one in which 256 total hosts exist. Of course, we may chose to take those 256 hosts and subent it, but in the end subnetsXhosts always equals 256. That being said, there is no way to take a class B address and create 8 class C addresses. That doesn't register with me. Niether answer provided in this dicussion seem viable.
On a class B address, 255.255.248.0 creates 32 subnets with 2048 hosts per subnet.
On a class B address, 255.255.224.0 creates 8 subnets with 8192 hosts per subnet.
We know a class C address supports a maximum of 256 total hosts. So how can a the two subnet masks above, be in consideration??? They create 2048 hosts and 8192 hosts respectively. Thats a hell-of-a-lot more than any class C address I have every seen.
After thinking about the question, the way it was worded in this forum, I cannot come up with any answer that makes sense. | |
| Yankee 2002-02-20, 7:56 pm |
| quote:
Originally posted by dmaftei
2032 and 2046 respectively. Why do you ask (since I'm sure you know the answer already)?! And what's the connection between this question and the quote?!
I asked because I have often heard a 255.255.248.0 loosely called 8 class C address blocks which of course agrees with the answer his book gave.
Yankee | |
| strikeattack 2002-02-20, 9:25 pm |
| I guess no one caught my MSFB joke. | |
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| I got it. Probably cos I set it up in the first place. Just for fun, I like to take regular acronyms and abbreviations and turn them into something humourous. A bit like a 'Spoonerism'. It relieves the boredom, and with the MFS of a B, it allows me to vent out now and again.
Hippo
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