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
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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