Thanks.
I think that I pretty well understand the technical aspects.
I know that I can use subnet -1 and subnet 0 in a Cisco environment
(with "ip subnet-zero").
My question was of a practical nature:
Does the CCNA 2.0 certification test assume that we can use 0 and -1
or does it assume that we cannot.
E.g., if encountered on the CCNA 2.0 cert test, what is the answer to
the following question:
Given the Class C network, 192.168.1.0, what mask is needed to
provide for 7 subnets?
The "real" answer (in the sense of what could be configured on the Cisco
routers and irrespective of any restrictions that hosts on those subnets
might have) would be 255.255.255.224, even without "ip subnet-zero".
The CCNA 1.0 answer would have been
255.255.255.240
What is the answer expected by CCNA 2.0 ? (Or maybe they scrupulously
avoid those particular questions :)
And, as I said, the ICND book still subtracts 2.
-------------------------------------------------
Tks | <mailto:[EMAIL PROTECTED]>
BV | <mailto:[EMAIL PROTECTED]>
Sr. Technical Consultant, SBM, A Gates/Arrow Co.
Vox 770-623-3430 11455 Lakefield Dr.
Fax 770-623-3429 Duluth, GA 30097-1511
=================================================
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Brian Lodwick
Sent: Tuesday, January 23, 2001 1:17 PM
To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
Subject: RE: CCNA 2 and subnets
Bob,
Howard answered this question for me a while back so I'll try to
answer it
for you now. This question is probobaly more in depth than you realize,
but
the question comes down to why did they used to say the equation for
finding
the amount of valid subnets is 2^#of hosts -2? And why now do we not -2?
Well the short answer is -we used to use Classfull addressing. With
classfull the reason we used the -2 was because it was a bad idea to use
the
all 0's or all 1's subnets(highly discouraged is I believe the
terminology)When an all 0's subnet update was sent to a classfull router
it
would not be able to decipher it from the entire network. This is
because in
clasfull the masks aren't sent with the updates therefore when the
classfull
mask is placed on say 192.168.0.0/28 it would change it to /24 because
again
the mask wasn't sent. Which would end up causing some issues obviously.
The
other one was the all 1's subnets. I'll just make an example. If you
think
along the same lines as the all 0's. Again in a classfull environment a
broadcast for a particular subnet would be interpreted as a broadcast
for
the entire network. 192.168.0.255/28 has different meaning than
192.168.0.255/24.
3Coms website has the best explaination I have found The article is
called:
Understanding IP addressing: Everything You Ever Wanted To Know by Chuck
Semeria.
Cisco, Microsoft, and the RFC's seem to dance around the topic.
>>>Brian
>From: "Bob Vance" <[EMAIL PROTECTED]>
>Reply-To: "Bob Vance" <[EMAIL PROTECTED]>
>To: "CISCO_GroupStudy List \(E-mail\)" <[EMAIL PROTECTED]>
>Subject: RE: CCNA 2 and subnets
>Date: Tue, 23 Jan 2001 08:24:37 -0500
>
>Yarrggh!
>Of course, that's
>
> (2^n) (*not* 2^(n-1) )
>
>Maybe there *is* something to that aspartame story ;>)
>
>-------------------------------------------------
>Tks | <mailto:[EMAIL PROTECTED]>
>BV | <mailto:[EMAIL PROTECTED]>
>Sr. Technical Consultant, SBM, A Gates/Arrow Co.
>Vox 770-623-3430 11455 Lakefield Dr.
>Fax 770-623-3429 Duluth, GA 30097-1511
>=================================================
>
>
>
>
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
>Bob Vance
>Sent: Monday, January 22, 2001 10:35 PM
>To: CISCO_GroupStudy List (E-mail)
>Subject: CCNA 2 and subnets
>
>
>Sorry for the lame question, but I gotta know :|
>
>We know that subnet -1 (all ones) is valid to config in IOS and that 0
>is OK with
>
> ip subnet-zero.
>
>For purposes of CCNA 2, do we assume that subnet 0 and -1 are valid,
>vs. CCNA 1 (where they were not) for questions like,
> "How many subnets can we have with this mask?
> "
>?
>Does the test make it clear in preliminary text?
>
>The archives seem to have conflicting answers.
>
>The Cisco Press ICND book (McQuerry, 1-57870-111-2) doesn't address the
>issue head on, but simply shows tables with (2^(n-1))-2 subnets.
>
>The Cisco Press 640-507 Cert Guide (Odom, 0-7357-0971-8) clearly says
>that 2^(n-1) is correct and yet points out that 0 is only valid with
>"ip subnet-zero" !
>
>Does anyone know the *definitive* answer for CCNA 2.0 ?
>
>
>-------------------------------------------------
>Tks | <mailto:[EMAIL PROTECTED]>
>BV | <mailto:[EMAIL PROTECTED]>
>Sr. Technical Consultant, SBM, A Gates/Arrow Co.
>Vox 770-623-3430 11455 Lakefield Dr.
>Fax 770-623-3429 Duluth, GA 30097-1511
>=================================================
>
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