The /64 is the prefix length. It is the number of bits in the address that is fixed. So a /64 indicates that the first 64 bits of the 128-bit IPv6 address are fixed. The remaining bits (64 in this case) are flexible, and you can use all of them. This means that when your ISP gives you a /64 they are giving you 264 addresses (that is 18,446,744,073,709,551,616 addresses).
IPv6 有几种常见的前缀长度。 /64 是单个 LAN 的大小。作为客户,您通常会从 ISP 获得 /56(256 个 LAN)或 /48(65,536 个 LAN)。然后,您可以创建多个 LAN,每个 LAN 都需要一个 /64。