The following table provides a convenient shortcut to all the CIDR prefix sizes, showing the number of addresses possible in each prefix and the number of prefixes of that size that may be numbered in the 32-bit IPv4 address space: notation addrs/block # blocks -------- ----------- ---------- n.n.n.n/32 1 4294967296 "host route" n.n.n.x/31 2 2147483648 "p2p link" n.n.n.x/30 4 1073741824 n.n.n.x/29 8 536870912 n.n.n.x/28 16 268435456 n.n.n.x/27 32 134217728 n.n.n.x/26 64 67108864 n.n.n.x/25 128 33554432 n.n.n.0/24 256 16777216 legacy "Class C" n.n.x.0/23 512 8388608 n.n.x.0/22 1024 4194304 n.n.x.0/21 2048 2097152 n.n.x.0/20 4096 1048576 n.n.x.0/19 8192 524288 n.n.x.0/18 16384 262144 n.n.x.0/17 32768 131072 n.n.0.0/16 65536 65536 legacy "Class B" n.x.0.0/15 131072 32768 n.x.0.0/14 262144 16384 n.x.0.0/13 524288 8192 n.x.0.0/12 1048576 4096 n.x.0.0/11 2097152 2048 n.x.0.0/10 4194304 1024 n.x.0.0/9 8388608 512 n.0.0.0/8 16777216 256 legacy "Class A" x.0.0.0/7 33554432 128 x.0.0.0/6 67108864 64 x.0.0.0/5 134217728 32 x.0.0.0/4 268435456 16 x.0.0.0/3 536870912 8 x.0.0.0/2 1073741824 4 x.0.0.0/1 2147483648 2 0.0.0.0/0 4294967296 1 "default route" n is an 8-bit decimal octet value. x is a 1- to 7-bit value, based on the prefix length, shifted into the most significant bits of the octet and converted into decimal form; the least significant bits of the octet are zero. Source: http://www.bgp4.as/