CIDR notation

A block of Internet IP Addresses can be shown in a compact notation by adding a "mask" to the standard "dotted quad" notation for 32-bit IPv4 addresses.

192.168.33.32/28

The /28 in the ip4 terms is CIDR notation for a block of 16 addresses. The number 28 is the number of bits in the "mask", which is a bit pattern used in testing address ranges. Comparisons using a mask are faster than a general integer range test, but the blocks are limited to sizes that are a power of two (/32=1, /31=2, 4, 8, 16, ... /24=256, ... /0=2^32).

CIDR notation is commonly used in routers, where speed is critical.