Configuring the CIDR value for a network address, as used, for example, on Filezilla server for IP address filtering.
Filezilla server requires, for the IPO address filtering, that one or more entries be added based upon the CIDR notation, for example: 192.168.1.123/16
Where to the left of the slash is the IP address range and to the right is the mask which determines which numbers are changeable.
8 bits are required to define each of the 4 numbers making up the IP address. Enabling each number to be 0 to 255.
The mask is derived from the number of bits used, counting from the left. This is used to define the bits which will be static. A lower mask value will allow a higher number of IP addresses through the fiter.
I had found that for those accessing the server the first set of octal numbers were static. The other three sets were the ones which were changing.
For our network number
IP Address:192.168.1.123
Netmask: 255.0.0.0
In binary: 11111111.00000000.00000000.00000000
This gives the CIDR value of 192.0.0.0/8
The table below shows the CIDR notation, the number of addresses passed and the network mask
| CIDR notation | Addresses Passed | Net mask | Comments |
|---|---|---|---|
| /0 | 4,294,967,296 | 0.0.0.0 | Every address |
| /1 | 2,147,483,648 | 128.0.0.0 | |
| /2 | 1,073,741,824 | 192.0.0.0 | |
| /3 | 536,870,912 | 224.0.0.0 | |
| /4 | 268,435,456 | 240.0.0.0 | |
| /5 | 134,217,728 | 248.0.0.0 | |
| /6 | 67,108,864 | 252.0.0.0 | |
| /7 | 33,554,432 | 254.0.0.0 | |
| /8 | 16,777,214 | 255.0.0.0 | |
| /9 | 8,388,608 | 255.128.0.0 | |
| /10 | 4,194,304 | 255.192.0.0 | |
| /11 | 2,097,152 | 255.224.0.0 | |
| /12 | 1,048,576 | 255.240.0.0 | |
| /13 | 524,288 | 255.248.0.0 | |
| /14 | 262,144 | 255.252.0.0 | |
| /15 | 131.072 | 255.254.0.0 | |
| /16 | 65,536 | 255.255.0.0 | |
| /17 | 32,768 | 255.255.128.0 | |
| /18 | 16,384 | 255.255.192.0 | |
| /19 | 8,192 | 255.255.224.0 | |
| /20 | 4,096 | 255.255.240.0 | |
| /21 | 2,048 | 255.255.248.0 | |
| /22 | 1024 | 255.255.252.0 | |
| /23 | 512 | 255.255.254.0 | |
| /24 | 256 | 255.255.255.0 | |
| /25 | 128 | 255.255.255.128 | |
| /26 | 64 | 255.255.255.192 | |
| /27 | 32 | 255.255.255.224 | |
| /28 | 16 | 255.255.255.240 | |
| /29 | 8 | 255.255.255.248 | |
| /30 | 4 | 255.255.255.252 | |
| /31 | 2 | 255.255.255.254 | |
| /32 | 1 | 255.255.255.255 |
