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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