Classless Inter-Domain Routing (CIDR)
CIDR is a packet routing system used to allocate IP address blocks. An IP address can be considered to have a network address and a host identifier. Both of these parts are calculated by the IP address and the number of mask bits.
CIDR notation is a standard syntax for writing IPv6 addresses with a routing prefix. It consists of an IPv6 address, a slash, then a number of bits to measure the routing prefix.
2002::1234:abcd:ffff:c0a8:101/64 is an example of IPv6 CIDR notation.