Type your input below. This will automatically do the calculation.
CIDR stands for Classless InterDomain Routing.
IPv4 CIDR notation: [IPv4 network address]/[host identifier]
.
For example: 192.168.0.1/24
Calculation
Let's calculate this: 192.168.0.1/24
Break it down, we have two parts: 192.168.0.1
and /24
.
The 192.168.0.1
is an IPv4 dotdecimal notation. The network address.
The /24
is the subnet mask as a host identifier (to denote its own network).
This is a neat explanation.
This is the mainline of how the tool above does the calculation. Steps:
 Convert the IPv4 dotdecimal notation to dotbinary notation.
192.168.0.1
►11000000.10101000.00000000.00000001
 Translate the subnet mask suffix, in above example:
/24
, into dotbinary notation.
The total length of the notation is 32 bits (32 of 0's and/or 1's).
Look at the number after the slash, it is 24. Meaning we create 24 of 1's from the left (24 leading 1bits). The last 8 bits (32  24 = 8) are filled 0's.
Now we have the binary version of the suffix.
Put dot (.) to separate each segment with 8 bits interval (dotbinary notation)./24
becomes11111111111111111111111100000000
, then11111111.11111111.11111111.00000000
 After that, do the
AND
operation for the binary version of the IPv4 and the binary version of the subnet mask.AND
OPERATIONS:
1AND
1 = 1
1AND
0 = 0
0AND
1 = 0
0AND
0 = 0
IP:11000000.10101000.00000000.00000001
Mask:11111111.11111111.11111111.00000000
AND
RESULT:11000000.10101000.00000000.00000000
 Then convert the
AND
result back to dotdecimal notation. This is the first IP within the block. The HOST address.11000000.10101000.00000000.00000000
►192.168.0.0
 We now wanna get the last IP within the block. To do that, first convert the subnet mask dotbinary notation to dotdecimal notation.
11111111.11111111.11111111.00000000
►255.255.255.0
 Then, do subtraction like so:
FULL MASK:
255.255.255.255
SUBNET MASK:255.255.255. 0

SUBTRACTION RESULT:0. 0. 0.255
Or, you could invert the subnet mask binary version, then convert it back to decimal.
11111111.11111111.11111111.00000000
inverted becomes00000000.00000000.00000000.11111111
00000000.00000000.00000000.11111111
to decimal is0.0.0.255
.
Same result. 
The last IP then is calculated by adding the subtraction result number 6 above with the first IP we got from step number 4.
STARTING IP:
192.168. 0. 0
SUBTRACTION RESULT:0. 0. 0.255
+
LAST IP:192.168. 0.255

COMPLETE.
So then192.168.0.1/24
is to tell the computer that its network HOST is at192.168.0.0
(and the network block is within192.168.0.0
—192.168.0.255
).
That's the main idea for translating the /number
suffix (network block) in IPv4 CIDR notation. It's not limited to this method, there are others, but the principles are like those above.
For more information about the subnet mask CLASS, IP TYPE, usable addresses within a block, preassigned addresses and things related to that, you could browse networking forums / websites / articles on Wikipedia.