The Wikipedia link is here Classless Inter-domain Routing
And here is the basic information you need to know.
A CIDR identifies a machine connected to a network.
An example CIDR for IPV4 address looks like this
192.168.100.14/24
.
The numbers
192
,
168
,
100
,
14
are four octets. An octet represents
8
bits. We use four octets, so we are using
32
bits to represent the machine.
A bit can have two possibilities either
0
or
1
. So, an octet can have 256 (2 ** 8) values, starting from 0 and ending with 255, which means 4 octets can represent 4,294,967,296 possibilities (2 ** 32).
But, the number after the
/
puts a limit on this.
Before, I explain how the limit is applied, I'll have to explain what number
24
means. The number
24
could also have been represented as 4 octets. If we did, it would look like
255.255.255.0
. The number
24
is a compact way of representing these 4 octets. Are you wondering how these are equivalent? The value
255
when represented by a an octet (8 bits) looks like this
11111111
. So
255.255.255.0
would be
11111111.11111111.11111111.00000000
. I put
.
in between for readability. As you can see, this big number has
24
leading bits set to
1
. That's the link between the two representations.
This number is telling us that while using the address
192.168.100.14
, don't use the first 24 bits or the first 3 octets. That is,
192.168.100
are off limits. That leaves us with only the last octet, so only 256 possible values :( ! Imagine if this number was
31
, that leaves only
1
bit or two possibilities.
The 3 octets that we do not use, along with a a
0
for the fourth octet
192.168.100.0
represents something called a
routing prefix
. If you represent this routing prefix in the form
255.255.255.0
, it's called the
subnet mask
and logically represents the same thing.