Question related to Classful IP Addressing

I cannot believe that I'm still seeing questions about how to manage class-based routing coming in to ServerFault:

and, of course this one:

There's just some I found very quickly on the first page of a google search.

Most of these (apart from the last one) are because people just don't understand CIDR routing or are living in 1994. If you're 56 years old and have lived under a rock for the last 20 years - fine. What disgusts me is that some of these people are students and they are clearly being TAUGHT THIS AT SCHOOL. And not just as a theoretical exercise, but AS FACT.

I went to a pretty crappy university where in 2004 our lecturer tried to teach us about setting an IRQ on a network card in an ISA BUS (she somehow managed to find enough old computers for our tutorials so we actually had to do this!) and our database lecturer taught me nothing useful in two semesters except how to understand the thickest russian accent I've ever heard. But even we didn't learn about class-based IP addresses.

Should we bother answering these questions, or should we have one master "Class based IP addresses are DEAD and here is why" question/answer that we can link to and hit the close button?

  • 2
    BTW have you seen this question. It may be a good candidate for a CIDR question. serverfault.com/questions/12854/cidr-for-dummies
    – Zoredache
    Commented Jul 28, 2010 at 6:17
  • @Zore, I just saw it then when you bumped it to the top. It's very good - I had never seen it before. It's even got a great title! Commented Jul 28, 2010 at 6:25
  • .. where is the <blink> tags on the new! :-D
    – Zypher
    Commented Aug 6, 2010 at 3:07
  • @Zyph, that better? ;) Commented Aug 6, 2010 at 3:10
  • oh that made my day :)
    – Zypher
    Commented Aug 6, 2010 at 14:44

5 Answers 5


Per Zoredache's answer:

Most new subnetting questions I just link to Evan's post

Is there a tag/list/something to denote the collection of question-answers that are the authoritative examples on their respective topic? Outside of subnets/CIDR, of course, those two seem pretty well covered. :)

As SF grows I suspect duplicate/beginner/one-hit-wonder questions will become a much larger issue. Does the SE engine include a method to organize and publish such example questions in an easier-to-find fashion?[1] Closing a question as duplicate/already-answered/"beginner" and redirecting the the One True Example Question may help. Providing new users a Sysadmin-FAQ But-Not-The-Site-Usage-FAQ could be useful.

[1] - I realize this can all be mitigated with a little searching+rtfsf.

  • +1 "Closing a question as duplicate/already-answered/"beginner" and redirecting the the One True Example" Commented Jul 28, 2010 at 13:38

The sad part is when i was in tech school (high school tech school not those crappy vocation schools that pretend to be college) (graduated a little under 10 years ago in 01) Class based IP addressing was taught as THE way to do subnetting ... if i remember we had 3 different tests on it. So I'm not surprised they are still teaching it. Heck I hadn't even heard of CIDR till i took a CCNA course 2 years later.

As for Class based IP addresses are dead, i would suggest an edit to Subnetting Wiki that explains class based addressing is dead as the master question.

  • I wasn't feeling bold enough to rewrite the entire structure of the question, so I added it as the first sub-question.
    – Andrew
    Commented Jul 28, 2010 at 2:01

I support the idea of a new question / answer about classful vs. cidr. I'm not so sure adding this to the subnetting wiki is such a great idea. It seems to be a rather large post already. Of course, I'm still kinda newbish.


The problem with classful routing is that I have a CCNA study book from 2005 that discussion classful subnetting. I would not be surprised at all if some colleges still teach it.

We were interviewing for a Network Support position just recently (~ sage II). Four out of the six candidates we interviewed where not able to tell us what CIDR or VLSM was.

Classes are dead, but I think we will probably see question about it until the majority of the Internet has switched to IPv6.

Most new subnetting questions I just link to Evan's post

  • You mean the classes are dead, not CIDR (in the IPv4 world). Also, I would expect a CCNA to know a little history of IP; but there's no reason to teach it in college (except maybe a 400 level Computer History class).
    – Chris S
    Commented Jul 28, 2010 at 12:31
  • @Chris S, fixed. The problem is that the CCNA book I was reading didn't cover it as if it was history, but instead the standard way things are done.
    – Zoredache
    Commented Jul 28, 2010 at 16:07
  • You know, this kind of thing is probably prime material for a Tag-wiki.
    – sysadmin1138 Mod
    Commented Aug 6, 2010 at 4:07

A few years ago I decided to do some Microsoft exams and get some certification to help switch jobs. As part of that process I obtained a few video training courses, most notably those from CBT, to help me revise and fill in those blanks resulting from things I didn't use day to day. Several of those courses still used, or at least refered to, class based addressing. So do a number of fairly recent texts I've seen from people/organisations that should know better.

The point I'm trying to make here is that, whether we like it or not, this stuff is still being taught, so we are going to continue seeing those questions.

As an aside, serverfault.com/questions/12854/cidr-for-dummies is a great example of a question that got shoved onto SU when it should have been put out of its misery.

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