Similar to borrowing bits from the host portion of an IPv4 address, with IPv6 bits can be borrowed from the interface ID to create additional IPv6 subnets. This is typically done for security reasons to create fewer hosts per subnet and not necessarily to create additional subnets.
When extending the subnet ID by borrowing bits from the interface ID, the best practice is to subnet on a nibble boundary. A nibble is 4 bits or one hexadecimal digit. As shown in the figure, the /64 subnet prefix is extended 4 bits or 1 nibble to /68. Doing this reduces the size of the interface ID by 4 bits, from 64 to 60 bits.
Subnetting on nibble boundaries means only using nibble aligned subnet masks. Starting at /64, the nibble aligned subnet masks are /68, /72, /76, /80, etc.
Subnetting on a nibble boundary creates subnets by using the additional hexadecimal value. In the example, the new subnet ID consists of the 5 hexadecimal values, ranging from 00000 through FFFFF.
It is possible to subnet within a nibble boundary, within a hexadecimal digit, but it is not recommended or even necessary. Subnetting within a nibble takes away the advantage easily determining the prefix from the interface ID. For example, if a /66 prefix length is used, the first two bits would be part of the subnet ID and the second two bits would be part of the interface ID.