Larry, I'm only beating this dead horse for the CCNA/ beginner types on the list, who really do need to learn to distinguish between the function of a route summary versus the practicality of subnetting classful networks such that subnet zero becomes an issue.
while I wouldn't bet the house on it, I suspect that even on Cisco tests, there would not be question about summarization where the subnet zero, all ones subnet count ( 2^n-2) would be the right answer. ( although I would not be surprised to see this in some of the study materials, given what I know about how quality control is valued with certain publishers. ) I was more concerned that it became a point of disagreement during the course of this thread, indicating that there were some who did not understand the "why" of things. Is ip subnet-zero enabled by default now? Which IOS release? Last I checked ( command reference for 12.2 ) the default was still "disabled" IP classless is now enabled by default, but not subnet-zero. this could have changed. the docs on CCO tend to be a bit behind reality. Chuck ""Roberts, Larry"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > I don't think its unreachable. Cisco tests OTOH.... > > Most people use the 2^n-2 rule for determining the number of > available/usable networks. The -2 is there because of the need to remove the > network and broadcast subnet. I don't write the rules, I just question them > :) > > I agree with you 100% that there are 16 subnets. I was trying to point out > that those that we bringing up the fact that in the "old world" only 14 were > available were not reading the question correctly. It didn't ask for > usability, it asked for quantity. As I am sure you are aware, as you take > more and more Cisco tests, it becomes important to clarify what they are > actually asking for, not what would make sense for them to ask for... > > Its also important to note that IP subnet zero is still needed on Cisco eq > for it to route/subnet properly, they just enabled it by default now... > > > Thanks > > Larry > > > -----Original Message----- > From: Chuck [mailto:[EMAIL PROTECTED]] > Sent: Friday, July 12, 2002 3:13 PM > To: [EMAIL PROTECTED] > Subject: Re: Class C summarization question [7:48367] > > > ah, but that is old world classful thinking. > > as per RFC 1812, effectively there is no such thing as subnet zero any > longer. per that RFC, all routers SHOULD route to any address in the form of > network:host, or network:subnet:host. > > subnet zero is a holdover from the old world. it is there because of the > concern that there is still so much old world equipment out there. RFC 1812 > is dated June 1995, and one wonders how long it takes in practical terms for > all manufacturers and all software stack writers to get all their stuff up > to standard.. not to mention how long it takes for the old stuff to be > removed from production. > > hhhmmmmm..... a brief look through ARIN seems to indicate that assignments > are not made out of "subnet zero" space > > but that is still a different question. a summarization produces a single > route where several existed before. if you see a summary 192.1.0.0/16, why > would you think that 192.1.0.0/24 is unreachable? > > Chuck > > > ""Roberts, Larry"" wrote in message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > Just to jump in late on this, but... The question doesn't ask how many > > of those class "C" are usable, which would be dependant on subnet O, > > but instead the question was how many you would be able to summarize. > > A /20 would in fact summarize 16, 14 of which are useable without > > subnet zero... > > > > > > Thanks > > > > Larry > > > > > > -----Original Message----- > > From: Dain Deutschman [mailto:[EMAIL PROTECTED]] > > Sent: Wednesday, July 10, 2002 4:05 PM > > To: [EMAIL PROTECTED] > > Subject: Re: Class C summarization question [7:48367] > > > > > > Hey everyone, > > > > Thanks for all of your help. I have decided that 16 must be correct > > since > it > > makes perfect sense and most of you back that up as well. I think the > > test question was just plain wrong. Anyway...I passed the CCNP Routing > > exam > today > > so I'm pretty happy. : ) Groupstudy is a great learning resource. > > Thanks everyone. Dain. > > > > ""Dain Deutschman"" wrote in message > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > I'm confused about a practice question for BSCN that I came across: > > > > > > Your routing tables are getting very large and you need to configure > > > route summarization. How many class C internet addresses can you > > > summarize with > > a > > > /20 CIDR block? > > > > > > Answer: 8 > > > > > > Would it not be 16? Where am I going wrong? > > > > > > -- > > > Dain Deutschman > > > CNA, MCP, CCNA > > > Data Communications Manager > > > New Star Sales and Service, Inc. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=48737&t=48367 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
