Prefixes in IPv6

CIDR survived the upgrade: IPv6 prefixes work exactly like IPv4 ones, just on 128 bits. The overwhelmingly common subnet size is /64: the first four groups name the network, the last four identify the machine.

With 64 host bits, a single /64 holds 18 quintillion addresses, so IPv6 planning stops counting hosts entirely. You count networks, and every LAN simply gets a /64.

The network prefix is written like a subnet: keep the network groups, compress the zero host part to ::, add the length.