Talk:Classless Inter-Domain Routing

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Prefix/suffix[edit]

I'm not quite sure here, but shouldn't "prefix" be replaced with "suffix" instead? --Maik

I think the usual terminology is prefix, since you route packets destined for 193.137.7.30 (for example) through the shortest path to the 193.137.7/24 prefix (which itself is aggregated and reachable through the 193.136/15 prefix (which is composed of the 193.136/16 prefix and the 193.137/16 prefix)).


The term prefix makes sense when you consider how the netmask is expressed in binary Robertbrockway 04:40, Apr 18 2005 (UTC)


I added a new diagram which should help clarify this.

Baccala@freesoft.org 06:31, 9 November 2005 (UTC)

Confusion[edit]

How in the heck did I interpret 4 minutes as 3 months??!? - Lucky13pjn 19:48, Mar 14, 2005 (UTC)

Example[edit]

The example about the /22 mask lists a Class-C address (192.168.0.0) but the default mask for Class-C is 255.255.255.0 or /24. I thought that you could only add ones bits to the default mask, not take them away. Wouldn’t a /22 mask have to be associated with a Class-A or B address?

This is the example given:

“192.168.0.0 /22 could be written 192.168.0.0 255.255.252.0”

Should it have been something like:

“172.168.0.0 /22 could be written 172.168.0.0 255.255.252.0”

--addnet 18:51, 23 July 2005 (UTC)


The restriction on taking bits away from the default mask only applied in the classful networking scheme. Classless routing is just what its name implies — you really don't have any restrictions anymore on what can be done to a "class A" or a "class C" address. With the exception of class D/E, the entire address space is now administered uniformly throughout. So there are no more "classes". Really.

Baccala@freesoft.org 06:29, 9 November 2005 (UTC)


Todd Lammle in his book CCNA: Cisco Certified Network Associate Study Guide, Fifth Edition ( San Francisco, London: Sybex, 2005) clearly states on page 109 that you cannot change the default mask. The address space is still classed because of class A, B and C network/broadcasting addresses. So a Class C network cannot have a submask shorter than /24 because the X.X.X.0 is the network address for it.

81.20.147.194 17:08, 25 February 2007 (UTC)

Origins of CIDR[edit]

The first person to suggest using a net-mask to cover a group of classful networks (as opposed to subnets of a single classful network) was Carl-Hubert Rokitansky (affectionately known as "Roki" to all :-), in his "Cluster Networking" proposal (which long predated CIDR). (See this message and this reply to the internet-history mailing list.)

I had thought for a long time that there were no original documents on line for this proposal; web searches revealed nothing. However, I have just discovered that the online copy (large [[Portable Document Format|pdf] file) of the Proceedings of the '1st' IETF (there was actually at least one earlier one) includes a copy of Roki's handwritten slides on the matter, on pp. 45-61 (pdf page numbers). Just noting this here for informational purposes! Noel (talk) 16:11, 16 September 2005 (UTC)

I always thought the story of the Jan 16, 1986 meeting as the "first" one was the only version... but since it's Noel talking, I suppose not... Noel, can you correct the story on IETF? --Alvestrand 17:06, 16 June 2006 (UTC)

Order, Order[edit]

I like the way you use a notation ie : " ... sixteen contiguous /24 networks ... " which is meaningless until you read later what the / means. Maybe you could assume the reader is new to the ideas of Classless Inter-Domain Routing, which is why he/she came to the page.

Similarly " ... The class, and hence the length of the subnet mask ...", before explaining what a subnet mask is. Very odd. I assume you are writing this for the people who already know it all, and just need a reminder?


Point taken. I think it's better now.

Baccala@freesoft.org 06:30, 9 November 2005 (UTC)

External links[edit]

There are plenty of online network calculators, and the two ones in the external links are quite limited in functionalities. How about adding this one. --Olivier Debre 07:45, 14 April 2006 (UTC)


And how about adding this: http://vlsm-calc.net/. 80.250.189.67

Both of these are not really CIDR Calculators. A CIDR calculator needs to tell you if your CIDR is valid and then do the expansion and show the mask the range and stuff. These two are subneting calculators. The second does not really work. The first would be OK for the subneting article but someone can already see all these tables in the article . I suggest http://ipduh.com/ip/cidr a calculator actually used by people who use CIDR notation at work. Tenretnieht (talk)

Assignment_of_CIDR_blocks[edit]

On the number available on the 208.130.28.0/22 block, it states it is just above 1000. I was about to precisise it to 1022 (ie, 2^10 - 2), to addresses lost to multicast and to loopback, but I wanted to place the suggestion here for a while, just to check if I am wrong. Comments welcome. -- Heptor talk 19:55, 17 May 2006 (UTC)

A little plain as it lacks discussion of drawbacks[edit]

I do believe there has to be something we sacrificed after CIDR from the original Internet. After all, there is no decision in life that doesn't have opportunity cost. I do believe Internet is less resilent to failure due to the use of CIRD. See, before CIDR a fault would take down a couple of networks, now it can take out countries service. Such info would improve(make the article stand out) the article as its rarely discussed in other written work

Could you elaborate a bit on this? It seems to me that in the "old days" of the Internet, when you had an entire Class C allocated to a user with ten hosts, that it was less resilient than if you take that traditional Class C space and chop it up into more useful and flexible chunks, which CIDR allows. CIDR, in and of itself, won't take down a country's service (this is what I think you need to expand upon). A bad implementation of BGP, on the other hand, could do this. Akamantauskas 21:10, 31 January 2007 (UTC)

VLSM?[edit]

CIDR and VLSM are basically diferent topics. whenever I ask for VLSM, it automatically directed to CIDR. Whats the matter ?????????????????

Sushil Kumar, India

(moved new discussion to bottom of page) Someone must have redirected VLSM to point here. I don't know what VLSM is intended to be an acronym for, so I don't know if it's right or not. If you want to, just click the "Redirected from" link and start editing. --Alvestrand 16:18, 16 June 2006 (UTC)
Variable Length Subnet Mask(ing) Luteijn 23:08, 11 July 2006 (UTC)



CIDR and VLSM are very different, but closely related.
  • CIDR is used as a form of slash notation as it relates to the borrowing of bits.
  • Variable Length Subnet Mask(ing) has to do with the ability to use differnt network/subnet values (represented by CIDR) in multiple locations of the same network.
As they they are different but related ; I changed redirect from pointing to whole page ( which makes illusion to the reader that they are the same thing ) , to redirecting to a subtopic called VLSM . I tried to add comparision , but is far from being complete or accurate . Melnakeeb (talk) 13:56, 7 March 2010 (UTC)

Regarding Prefix[edit]

I am cofused whether the prefix would be no. between 0 to 32 or 0 to 31!!

The numbering is from 0 to 31, as IP addresses are 32 bits in length, and there are 32 numbers between 0 and 31 (count 'em). If you did 0 to 32, you would end up with 33 total numbers. Akamantauskas 21:06, 31 January 2007 (UTC)
The prefix can be 0 if referring to everything (ie, all hosts) -> for example, the default route 0.0.0.0/0, or 32 if referring to only one specific host -> for example 192.168.0.1/32, so when the prefix length is 32, there is only one address. Neo-Vortex (talk) 22:02, 7 September 2010 (UTC)

the merge thing[edit]

I suppose I'd be in favor of merging in supernet and Provider-based addressing, mostly because those articles are not only poorly written, but inaccurate; having them go away in lieu of the content here seems like a good thing. The subnetwork article also needs an awful lot of work to make it jibe with reality, but I don't think that it'd be correct to merge it in here. --moof 05:33, 4 December 2006 (UTC)

I'm opposed to merging Classless Inter-Domain Routing into Provider-based addressing since CIDR is by far the more commonly used term (of course, this is only from my personal experience). Perhaps doing the opposite and folding Provider-based addressing as a subsection into CIDR would be a better idea? Scraimer 00:08, 16 February 2007 (UTC)

Can i just say that if wikipedia is going to put itself at the top of google searches then these explanations need to be easier to understand. The first paragraph should either explain in laymans terms or refer me to another page that will explain some context in laymans terms . I just read the term CIDR in reference to storing IP addreses in databses. I know what an IP address is at a high level , I know what a database is - I still have no idea what CIDR is after reading this article. If you cant tell me, tell me where i can find it. —Preceding unsigned comment added by 203.208.238.16 (talk) 16:14, 13 January 2008 (UTC)

Provider-based addressing is a different concept from CIDR. The various geo-based addressing schemes that have been proposed over the years for IPv6 are all CIDR-based, but they are not provider-based. --Alvestrand (talk) 02:57, 2 January 2009 (UTC)

Confusion[edit]

In the part about assigning, it says that 208.128.0.0/11 is part of 208.130.29.33. How can 208.128 be part of 208.130? I can understand their both being 208s, but wouldn't the second part mean that 128 and 130 are different subnets of 208?

If someone can explain that in better detail and put it into the article it would be of great help. —Preceding unsigned comment added by 146.145.251.34 (talk) 13:56, 7 December 2010 (UTC)

CIDR notation merge[edit]

This seems like an easy merge. I don't see a reason to have a separate article for the notation. --Kvng (talk) 20:25, 8 March 2011 (UTC)

Oppose:This is like trying to merge Internet Protocol and IP address.Jasper Deng (talk) 00:47, 9 March 2011 (UTC)
Weak Oppose: While the relationship cited by previous commenter is poor--IP and IP address are not analogous to this issue--I do think there is justification to separate them for sake of convenience. I used to think differently when the article was first created by WRS, but I am fine with separation as it has proven useful. The separate article, provides a mean to very briefly explain the concept, which would be somewhat of duplication of content within the larger article, where the same ideas are presented in broader context. Kbrose (talk) 16:30, 11 March 2011 (UTC)
Support: I think it's an easy merge and easy decision to merge. The notation article, necessarily, has to introduce the concept of CIDR. The CIDR article discusses notation, of course. The duplication of material means that it's twice the work for editors and, worse still, the two separate descriptions can get out of sync and cause all sorts of trouble. ... Keep in mind that if they're merged then the "CIDR notation" entry can become a re-direct straight to the appropriate section in "Classless Inter-Domain Routing". Kace7 (talk) 19:26, 10 May 2011 (UTC)
Support: 95.144.92.16 (talk) —Preceding undated comment added 21:09, 19 August 2011 (UTC).
Support: This could also include examples as discussed below. I think people looking for CIDR will be helped by having a single article.--Flexdream (talk) 12:27, 5 March 2012 (UTC)
Support: YBG (talk) 05:09, 7 March 2012 (UTC)
I've added a merge tag to this article -- somehow it was added to the other article but not to this one. YBG (talk) 06:18, 7 March 2012 (UTC)
The tag was removed without explanation by Kbrose (talk · contribs) on 2011-12-12. --Kvng (talk) 15:15, 9 March 2012 (UTC)
Support: Merging the articles also addresses one of the concerns on the CIDR Notation article for insufficient explanation. -- Dave Braunschweig (talk) 04:18, 4 December 2012 (UTC)
Support: KShiger (talk) 18:25, 8 December 2012 (UTC)
Support: Notation article will never grow beyond a stub and it can't really be understood without reading the main article. Chuunen Baka (talkcontribs) 16:31, 20 December 2012 (UTC)
Comment - CIDR notation can also be used to describe classful network designs too, so I still oppose this merge.--Jasper Deng (talk) 20:24, 5 January 2013 (UTC)
Oppose - per Jasper Deng. It's a Fox! (Talk to me?) 02:15, 1 February 2013 (UTC)

Yes check.svg Done there appeared to be reasonable consensus to merge and no strong reasons not to. -—Kvng 18:39, 1 March 2013 (UTC)

Examples[edit]

I think it would be useful to add some examples to this article. For example, how to target one IP, or what is the equivalent of 123.54.65.*, etc. That would make the article more useful and perhaps easier to understand for non-specialists. Laurent (talk) 14:53, 11 May 2011 (UTC)

CIDR Calculator[edit]

Adding http://ipduh.com/ip/cidr as a CIDR math calculator — Preceding unsigned comment added by Tenretnieht (talkcontribs) 12:10, 26 March 2012 (UTC)

Reporting this addition here does not make it anymore credible or useful. WP is not a link farm. Howto and calculator links are not considered to be appropriate content. Insistence on this in this case gives the impression of WP:COI, trying to drive traffic. Kbrose (talk) 14:53, 26 March 2012 (UTC)

Do you really consider this CIDR calculator irrelevant and not useful to the article? Could you backup this? I am saying that this CIDR calculator is both relevant and useful for this article both the examples and as an ops help tool. ( I wrote a large part of this article and I consider myself to be more qualified to talk on this and some other CS subjects than you ) I actually do use CIDR notation every day. I am going ahead to add to the link again. I am not going to accept your edit. Plus I am fluent in Greek and what you are saying on your page sounds silly. {worthy to be a king if he has ἠθικός too -- You either going to write 'worthy to be a king if he IS ἠθικός too' or 'worthy to be a king if has ἠθος too' or leave it like the way you have it so people who know Greek can laugh on you }. I have told you before how to correct it , but it is evident that you have issues accepting advise on stuff you don't know about. You have lost all credit with me. If someone else wants to edit my stuff OK, If someone else wants to remove this link or whatever I ever contributed it's fine --but not you!Tenretnieht (talk)

Please familiarize yourself with Wikipedia:What Wikipedia is not, Wikipedia:External links, WP:ELNO. Kbrose (talk) 16:12, 26 March 2012 (UTC) (updated)
I read it. Could you explain to me how adding the CIDR calculator link on this article relates to Wikipedia:NOTHOWTO? And then how is the CIDR calculator not related to this article or why it is not useful to the people trying to figure out what CIDR is and how it works by reading this article? Tenretnieht (talk)
If you keep on changing stuff you already said our conversation becomes very difficult to follow by others. Even us. I believe I have read all of these before. I am going to read them again. Also I would perceive your response as "There is nothing against my edit on Wikipedia:NOTHOWTO." However, You do need to make a point Kbrose. When I corrected your Greek above I did correct your Greek and explained why. Not just above but the first time. I did not point you to the Ancient Greek Grammar , the Modern Greek Grammar, two enormous lexicons , and a Greek Syntax Manual and said please familiarize yourself with them. And if you were insisting on proof I would give you book title, page number , paragraph number etc . Also, Next time you need to say something please say it under of what I just said.Tenretnieht (talk)
We try to say what CIDR prefixes are, but not how to calculate them; therefore, the calculator link lends nothing more to this article.Jasper Deng (talk) 02:08, 27 March 2012 (UTC)
The CIDR calculator Does Not calculate CIDRs. It Checks - Corrects - and Explains CIDR notation. It is a CIDR expert Tutor on Demand therefore it is related very much to CIDR notation. Tenretnieht (talk)
For example try to explain 127.0.43.0/16 to someone who is reading this article and he is not familiar with CIDR notation or not familiar enough with CIDR notation and the Internet Structure. The CIDR calculator would simply say http://ipduh.com/ip/cidr/?127.0.43.0/16 . Once you are done explaining this to someone who is reading this article and happens to wonder if 127.0.43.0/16 is a correct or wrong CIDR , try to explain to him and the rest of the readers, the next 2^32*33-1 possible combinations they may wonder about, correct them if wrong , and explain where they should or should not be used. Tenretnieht (talk)
If your answer is that this is not wikipedia's job , then you should not be here. Tenretnieht (talk)
If your solution is to make a table with all CIDRs, whether if it is correct or no , corrections if needed, their expansions, their explanations, their masks ,where they should be used or no and special uses, THEN please leave the CIDR calculator link here for the next 3 lives it will take you to make the table , and then you will still have to give us some way of looking up CIDRs fast in your table. Tenretnieht (talk)
If your solution is to put Kbrose on some irc channel saying to the users who may wonder about one of the 2^32*33 CIDRs to familiarize themselves with the article , 150 RFCs , and binary system arithmetic please do not even bother to mention it. Tenretnieht (talk)
If you have some other CIDR calculator - CIDR explanator in mind please point us to it.Tenretnieht (talk)
If you do not like the CIDR calculator because you do not like http://ipduh.com or because http://ipduh.com/ip/cidr shows an ad (1 out of the 3 times used i think ) then I could look into writing it in php and putting it on a tool server. However this would take me a while and it is duplicating job - reinventing the wheel and extra load to the tool server that will host it. Tenretnieht (talk)
http://ipduh.com/ip/cidr/ is a piece of software written by someone who is using CIDR every day to teach CIDR because he got tired of teaching, correcting , and explaining CIDR notation himself. Tenretnieht (talk)
Kbrose and jasper, I don't know how you are related. You may as well be the same dude. I do not care and I do not have anything against you. This is a debate. I do not mind your skepticism and I do not expect everyone to be an expert on these matters. I do not mind people like you, who 'police' wikipedia and I find them useful. I have to admit that I never had that much difficulty to add 3 lines to wikipedia. On this and another article. I just want you to know that I have good faith but strong opinions on matters I know well. If you let me I 'll make wikipedia a little better. Tenretnieht (talk)
We're different as oranges and apples. Besides the fact that it's not Wikipedia's job to link to these things, your conflict of interest with that site, at least what Kbrose said, is a factor here. I realize your frustration, but I'm not going to get into another hours-long argument with you when everything's so clear-cut. If you'd like I can ask for a third-party editor to comment, but I'm not spending more of my time on things like this.Jasper Deng (talk) 19:37, 27 March 2012 (UTC)
OK, we cannot reach agreement. I think that I better take this dispute to Wikipedia:External links/Noticeboard and https://en.wikipedia.org/wiki/Wikipedia:WikiProject_Computing/Task_forces/Networking . I assume we agree on this. Tenretnieht (talk) 23:44, 27 March 2012 (UTC)
There surely are 10s of thousands of all kinds of calculators on the net, easily accessible via any search engine, and people try all the time to add their favorite one to the WP networking articles. All of them are routinely removed when discovered, as WP is not a collection of how-to information and tools. Kbrose (talk) 23:48, 27 March 2012 (UTC)


>There surely are 10s of thousands of all kinds of calculators on the net < Could you point me to one that does what http://ipduh.com/ip/cidr/ does?
>easily accessible via any search engine < The contents of this article, all the RFCs, and all the EL on this article are relatively easily accessible at http://www.google.com/search?q=cidr as well.
If I use this argument as an axiom I can prove that wikipedia is useless.
>and people try all the time to add their favorite one to the WP networking articles All of them are routinely removed when discovered < says expert Kbrose
> WP is not a collection of how-to information and tools. < the CIDR calculator is not a howto is a CIDR notation teacher --a CIDR Notation tutor.
Apparently you cannot respond to any of my theses. Or maybe you did not even bother to read them.
Apparently you cannot understand the value of this link and we cannot reach agreement.
I am in the process of taking the dispute to Wikipedia:External links/Noticeboard and Wikipedia:WikiProject Computing/Computer networking task force. 00:24, 28 March 2012 (UTC)

Bad-faith edits by user Tenretnieht on 2012-03-28[edit]

The slew of edits by this user can hardly be interpreted any other way than retaliation for the disqualification of its prior edits, insisting on the display of a favored link to a web calculator. The user now has escalated this insistance to display it in numerous responses to years-old message here. This is raising the suspicion of WP:COI and bad-faith editing. I have therefore removed all modifications to this talk page on the date given by that user.

Another user has flagged the main article here with a tag of cleanup request for reason of "poorly written, poor grammar and writing style". The reason is clearly the additions by user Tenretnieht, which can only be described as a 'hack job' of poor judgement and lack of subject knowledge. For example, the user introduced language that stated "IPv6 is classless". The concept of a class feature makes no sense for IPv6 as it has never been defined what that would mean. The statement makes as much sense as saying "IPv6 is colorless", as the concept of color has never been defined in this context. VLSM was unwarrantably capitalized; VLSM cannot be construed as a proper noun, as it was a group of methods with a common theme, not a specific procedure, as is CIDR. After the user's edits, the article displayed an array of grammatical problems, poor construction and style, as noted already, and has technical problems.

Therefore, I have reverted the article to its state before these changes. The changes by this user were no improvement whatsoever. Kbrose (talk) 02:04, 29 March 2012 (UTC)

Kbrose, you just vandalized the article and the article conversation. None of your statements is true. You even deleted my votes on two matters and my answers to other WP users. Foo (talk) 23:03, 30 March 2012 (UTC)