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CIDR Cheat Sheet

June 21st, 2010

For those of you needing quick access to a CIDR/Dotted-decimal cheat sheet, we are providing you with the one below. This schema provides you with the CIDR notation, starting with /3, its Dotted-decimal counterpart, and the total number of available subnets within each network block:

CIDR Dotted-decimal Subnets
/3 224.0.0.0 536870912
/4 240.0.0.0 268435456
/5 248.0.0.0 134217728
/6 252.0.0.0 67108864
/7 254.0.0.0 33554432
/8 255.0.0.0 16777216
/9 255.128.0.0 8388608
/10 255.192.0.0 4194304
/11 255.224.0.0 2097152
/12 255.240.0.0 1048576
/13 252.248.0.0 524288
/14 255.252.0.0 262144
/15 255.254.0.0 131072
/16 255.255.0.0 65536
/17 255.255.128.0 32768
/18 255.255.192.0 16384
/19 255.255.224.0 8192
/20 255.255.240.0 4096
/21 255.255.248.0 2048
/22 255.255.252.0 1024
/23 255.255.254.0 512
/24 255.255.255.0 256
/25 255.255.255.128 128
/26 255.255.255.192 64
/27 255.255.255.224 32
/28 255.255.255.240 16
/29 255.255.255.248 8
/30 255.255.255.252 4
/31 255.255.255.254 2
/32 255.255.255.255 1

© 2010 – 2011, Stewart White. All rights reserved.

  1. Costas
    November 10th, 2010 at 14:09 | #1
    Reply | Quote

    Congratulations for the amazing work you are presenting us here. I am afraid I am relatively green in fully understanding the IP notation and I have a question:
    Suppose I wish to place a deny statement in my .htaccess file which includes a line for xx.0.0.0/16 or xx.0.0.0/24 or xx.0.0.0/32 what will be the range of IPs that will be denied in each one of the three cases. I am guessing that in the last one will be: xx.0.0.0 to 255.255.255.255 but I have the hunch that I am wrong.

    Thank you

    Costas – Toronto

    • Stewart White
      November 11th, 2010 at 12:45 | #2
      Reply | Quote

      each CIDR range you describe provides for a specific number of IP addresses
      For example: /16 = 65536; /24 = 256; /32 = 1 address/

      xx.o.o.o/16 = xx.0.0.0 – xx.0.255.255 (65536 IP addresses)

      xx.0.0.0/24 = xx.0.0.0 – xx.0.0.255 (256 IP addresses)

      xx.0.0.0/32 = xx.0.0.0 (1 IP address)

      Based on the above you could add deny statements as follows:

      deny from xx.o.o.o/16
      deny from xx.o.o.o/24
      deny from xx.o.o.o/32 (or just “deny from xx.o.o.o”)

  2. Dean Elliott
    July 29th, 2010 at 11:08 | #3
    Reply | Quote

    Nice work Stewart.

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