Understanding CIDR Subnet Mask Notation and note on private IP addresses source http://doc.m0n0.ch/quickstartpc/ 22 fŽv 2007 file CIDR subnet mask & private IP notation m0n0wall uses a subnet mask format that you may not be familiar with. Rather than the common 255.x.x.x, it uses CIDR (Classless InterDomain Routing) notation. CIDR Table You can refer to the following table to find the CIDR equivalent of your subnet mask. Table 1.1. CIDR Subnet Table Subnet Mask CIDR Prefix Total IP's Usable IP's Number of Class C networks 255.255.255.255 /32 1 1 1/256th 255.255.255.254 /31 2 0 1/128th 255.255.255.252 /30 4 2 1/64th 255.255.255.248 /29 8 6 1/32nd 255.255.255.240 /28 16 14 1/16th 255.255.255.224 /27 32 30 1/8th 255.255.255.192 /26 64 62 1/4th 255.255.255.128 /25 128 126 1 half 255.255.255.0 /24 256 254 1 255.255.254.0 /23 512 510 2 255.255.252.0 /22 1024 1022 4 255.255.248.0 /21 2048 2046 8 255.255.240.0 /20 4096 4094 16 255.255.224.0 /19 8192 8190 32 255.255.192.0 /18 16,384 16,382 64 255.255.128.0 /17 32,768 32,766 128 255.255.0.0 /16 65,536 65,534 256 255.254.0.0 /15 131,072 131,070 512 255.252.0.0 /14 262,144 262,142 1024 255.248.0.0 /13 524,288 524,286 2048 255.240.0.0 /12 1,048,576 1,048,574 4096 255.224.0.0 /11 2,097,152 2,097,150 8192 255.192.0.0 /10 4,194,304 4,194,302 16,384 255.128.0.0 /9 8,388,608 8,388,606 32,768 255.0.0.0 /8 16,777,216 16,777,214 65,536 254.0.0.0 /7 33,554,432 33,554,430 131,072 252.0.0.0 /6 67,108,864 67,108,862 262,144 248.0.0.0 /5 134,217,728 134,217,726 1,048,576 240.0.0.0 /4 268,435,456 268,435,454 2,097,152 224.0.0.0 /3 536,870,912 536,870,910 4,194,304 192.0.0.0 /2 1,073,741,824 1,073,741,822 8,388,608 128.0.0.0 /1 2,147,483,648 2,147,483,646 16,777,216 0.0.0.0 /0 4,294,967,296 4,294,967,294 33,554,432 So where do these CIDR numbers come from anyway? The CIDR number comes from the number of 1's in the subnet mask when converted to binary. The common subnet mask 255.255.255.0 is 11111111.11111111.11111111.00000000 in binary. This adds up to 24 1's, or /24 (pronounced 'slash twenty four'). A subnet mask of 255.255.255.192 is 11111111.11111111.11111111.11000000 in binary, or 26 1's, hence a /26. And so on... Private IPv4 address space (from the Wikipedia) RFC1918 name IP address range number of addresses classful description largest CIDR block (subnet mask) host id size 24-bit block 10.0.0.0 Ð 10.255.255.255 16,777,216 single class A 10.0.0.0/8 (255.0.0.0) 24 bits 20-bit block 172.16.0.0 Ð 172.31.255.255 1,048,576 16 contiguous class Bs 172.16.0.0/12 (255.240.0.0) 20 bits 16-bit block 192.168.0.0 Ð 192.168.255.255 65,536 256 contiguous class Cs 192.168.0.0/16 (255.255.0.0) 16 bits