I think he means on the exam only is it -2 or not for the subnet. I'd like
to know too since The two books say something different as Bob mentioned.
At 06:54 PM 1/23/2001 +0000, Brian Lodwick wrote:
>Using Cisco's website try to find out why the all 1's and all 0's subnet's
>are strongly discouraged in a classfull environment.
>
> >>>Brian
>
>
> >From: "Lowell Sharrah" <[EMAIL PROTECTED]>
> >To: <[EMAIL PROTECTED]>,<[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>
> >Subject: RE: CCNA 2 and subnets
> >Date: Tue, 23 Jan 2001 13:45:41 -0500
> >
> >cisco has a pretty good website out there too
> >
> >http://www.cisco.com/techtools/ip_addr.html
> >
> > >>> "Brian Lodwick" <[EMAIL PROTECTED]> 01/23/01 01:16PM >>>
> >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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