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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