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Home > Archive > CCNA > October 2000 > Subnetting with number of bits or a.b.c.d/# form
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Subnetting with number of bits or a.b.c.d/# form
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| The following two questions are from the ccna_studyguide CD from the Lammle 2nd Edition.
1. What is the valid host range of the follwing device? 172.16.10.33/27
ANSWER - 172.16.10.33-62
2. What is the broadcast address of the following subnet?
172.16.10.19 with 12 bits of subnetting
ANSWER - 172.16.10.31
In question #1 the 27 means 27 TOTAL bits, not RELATIVE to the class B address, so the mask would be 255.255.255.224.
In question #2 the 12 bits does not mean 12 TOTAL bits but 12 bits RELATIVE to the class B default mask. If it were 12 TOTAL bits the mask would be 255.240.0.0, which surely in not correct. But as 12 bits RELATIVE to the default mask the mask would be 255.255.255.240, which makes more sense.
So the question is what does Cisco mean when it says 172.16.10.33/12 or 12 bits of subnetting? Is it the TOTAL bits contained in the mask or the addition bits that are to the added to the default subnet mask. | |
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| It means just what you have discovered. There weren't many questions on subnetting, however it is important that you understand the ways of saying the same thing.
I think you got it!!
Reamer | |
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| As Reamer said, you got it. To give a slightly more detailed explanation...
slash notation (I _think_ that's what they call it) simply refers to the number of 1's in the netmask. Rather than writing 192.168.0.1 netmask 255.255.255.0 many books and webpages will simply write 192.168.0.1/24
meaning that there are 24 1's (in binary of course  ) in the netmask. This is standard, both with Cisco and others.
However, at least in Cisco exams, and Mr. Lammle's book, X bits of subnetting means in addition to the default mask. So you have address 172.16.0.5 with 5 bits of subnetting means the netmask has 5 ADDITIONAL bits, on top of the 16 bits of a default class B netmask.
Therefore, X bits of subnetting and a.b.c.d./x are two different things.
HTH
Scott |
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