Understanding IP class range.

[Kevin Fries]

I'm not trying to be argumentative, but your logic is misleading.  CIDR did
not create any more addresses than before CIDR became popular.  If anything
it decreased the potential addresses by eliminating the exotic non-linear
net masks.  As I said in my previous post... It comes down to the binary.
The binary does not change.  All addresses are binary, all masks are
binary, all switching is binary... All built on the work of George Bool,
and he rarely gets any credit.  CIDR or subnet mask are just two ways of
describing the exact same thing.  It's semantics.  Nothing is created or
lost in using either form.  What you call a /28 used to be referred to as
255.255.255.240, and /30 was 255.255.255.252, and your example of /31 makes
no sense at all... This is an unusable network as there are no room for
hosts ?!?!?

Kevin
On Aug 31, 2015 11:35 PM, "Michael Butash" <michael at butash.net> wrote:

> Really the /24 style of CIDR notation was meant to describe everything
> between the classful bits of a /8, /16, and /24, otherwise known as class
> a, b, and c when it simply wasn't good enough for "big, medium, small"
> sizing.  The internet *is* the in-between with CIDR blocks, why a full
> internet routing table consists of some ~540k routes of them.
>
> Old rules say if you were a big company like IBM, you got a /8. If you
> were medium, you got a /16.  Less, you got /24, but that didn't work too
> well once people realized there was a land-grab occurring for ipv4 space.
> CIDR notation addressed that to provide more subnet bits to work with.  The
> decimal "255.255.255.0" version you know and love just happens to be most
> common.
>
> Unless you deal with networks, really you just need to remember everything
> less than a /24 for the most part.
>
> It's mostly all ^2 and half-math really, programmed into my brain long
> since, but I remember it like this easiest:
>
> cidr      host addresses
> /24 == 256
> /25 == 128
> /26 == 64
> /27 == 32
> /28 == 16
> /29 == 8
> /30 == 4
> /31 == 2
> /32 == 1
>
> Why you might want to use a /31 or /32 are some of the more interesting
> nuances.  You can't talk on the internet directly with less than a /24 or
> you are mocked and summarily denied.
>
> There's always ccna books floating around, doesn't matter if 20 years old
> or 1 to get you started in the wonderful world of IP.
>
> -mb
>
>
> On 08/31/2015 09:03 AM, James Mcphee wrote:
>
>> I've always heard the /## notation referred to as cisco notation.
>>
>
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