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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