IPv6 - Network and Interface Identifiers
An IPv6 address is a 128-bit number that is separated into two components: the network identifier and the interface identifier. Take a look at the following diagram, where "N" is the prefix.
In the same way that an IPv4 address is separated into the network and host portion based on a subnet mask and IPv6 address is separated into the network identifier and the interface identifier based on the prefix.
An IPv6 address with a prefix of /64 will have a network identifier of 64 bits and an interface identifier of 128-64 = 64 bits. Similarly, an IPv6 address with a prefix of /96 will have a network identifier of 96 bits and an interface identifier of 128-96=32 bits.
The network identifier is predefined either using a DHCPv6 server or using stateless autoconfiguration, typically using NDP. The interface identifier can be determined in various ways, including EUI-64.