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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